Where can i put my home office on a Short term rental?

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@my.msiproperties  Short answer: Line 19 Other Expenses on Schedule E.
Enter on line 19 any ordinary and necessary expenses not listed
on lines 5 through 18.

To deduct home office expense.  your rental activities have to qualify as a “trade or business” under the tax law.

Under new rulings you’ll need regular and continuous involvement with your rental activities to meet this requirement. Most rental owners meet the criteria, even if you have only one rental property. The time your agent (e.g. property manager, employee, or virtual assistant) spends on your rentals counts to meet this requirement as well. 

To deduct expenses for business use of the home, you must use part of your home as one of the following:

1. Exclusively on a regular basis as your principal place of business for your trade or business;
2. Exclusively on a regular basis as a place where you meet or deal with your patients, clients, or customers in the normal course of your trade or business;
3. A separate structure that's not attached to your home, used exclusively on a regular basis in connection with your trade or business;
4. On a regular basis for storage of inventory or product samples used in your trade or business of selling products at retail or wholesale, so long as your home is the sole fixed location of such trade or business;
5. For rental use; or
6. As a daycare facility.

Your second hurdle is setting aside a physical space in your home that qualifies for the home-office deduction. For this to work, you need to use that space in your residence regularly and exclusively as the principal place of business for your rental activities. For example, the space can be an entire room or a portion of a room. But it cannot be your table or a shared office. Again, there are actual court cases to give us guidance in these details. A place where you use a filing cabinet exclusively for rentals though does take up space and does count!!! If the exclusive use requirement applies, you can't deduct business expenses for any part of your home that you use both for personal and business purposes.

Establishing a rental property home-office does a couple of important things to your household expenses: 

• Turns non-deductible personal household expenses into business tax deductions.

• Moves a portion of the household expenses normally deductible on Schedule A (only if you itemize) to your rental properties on Schedule E.

That second bullet point is especially important after the passage of the Tax Cuts and Jobs Act put a $10,000 limit on your Schedule A state and local tax deductions, and lowered the amount of your mortgage on which you deduct mortgage interest from $1 million to $750,000.

Regular method - You compute the business use of home deduction by dividing expenses of operating the home between personal and business use. You may deduct direct business expenses in full, and may allocate the indirect total expenses of the home to the percentage of the home floor space used for business. A qualified daycare provider who doesn't use his or her home exclusively for business purposes, however, must figure the percentage based on the amount of time the applicable portion of the home is used for business. Self-employed taxpayers filing Schedule C (Form 1040), Profit or Loss From Business (Sole Proprietorship) first compute this deduction on Form 8829, Expenses for Business Use of Your Home.

Simplified option - While taxpayers can still figure the deduction using the regular method, many taxpayers may find the optional safe harbor method less burdensome. Revenue Procedure 2013-13PDF allows qualifying taxpayers to use a prescribed rate of $5 per square foot of the portion of the home used for business (up to a maximum of 300 square feet) to compute the business use of home deduction. Under this safe harbor method, depreciation is treated as zero and the taxpayer claims the deduction directly on Schedule C (Form 1040). Instead of using Form 8829, the taxpayer indicates the taxpayer's election to use the safe harbor option by making two entries directly on the Schedule C for the square footage of the home and the square footage of the office. Deductions attributable to the home that are otherwise allowable without regard to business use (such as qualified residence interest, property taxes, and casualty losses) are allowed in full on Schedule A (Form 1040), Itemized Deductions. For more information, see Simplified Option for Home Office Deduction and FAQs – Simplified Method for Home Office Deduction.

Regardless of the method used to compute the deduction, you may not deduct business expenses in excess of the gross income limitation. Under the regular method for computing the deduction, you may be able to carry forward some of these business expenses to the next year, subject to the gross income limitation for that year. There's no carryover provision under the safe harbor method, but you may elect into and out of the safe harbor method in any given year.

https://www.irs.gov/forms-pubs/about-publication-587