HSA Contributions

First, verify with the employer how the money was deducted.  It could be that it was after taxes and thus not showing in box 12.  The amount is still deductible. You should get form 5498-SA that shows the contributions to your HSA.   When you go through the HSA interview you will get to the boxes to enter contributions.  The first box would be auto-populated from the W2 entries.  It should be blank if nothing is showing on the W2. The second box is where you would enter the contributions amounts shown on the form 5498-SA.   

The expert is mostly correct but you need some more information.

First, you definitely should determine whether the deductions were pre-tax or post-tax.  (If post-tax, the employer is costing themselves money, since they also save if the deductions are pre-tax.  If they are a small business, you might point them in the direction of an accountant or payroll service to explain it to them.)

Your HSA bank can verify the deposits made during the year.  They may keep a record of employer vs other deposits, or they may not.  But you probably won't receive form 5498-SA until May.  Since taxpayers can make retroactive 2019 deposits up to April 15, 2020, the bank probably won't mail the 5498-SA until after April 15, to make sure they capture all the 2019 deposits.  But, they still should have records you can use to verify the deposits now.

Then, if the deductions were after-tax, run the HSA interview and enter them when it says "did you make any additional deposits not through payroll" and you will get a tax deduction.

But if the deductions were pre-tax and not on your W-2, don't enter them here.  You will get an illegal second deduction.  Instead, ask your employer for a corrected W-2.  If they won't provide one, then run the HSA interview and look for a question that says something like "Did your employer tell you about any additional deposits that are not on your W-2 (this is unusual)."