It appears that the "management company" is the parent company of the "one company you worked for and received a W-2 from".
The "proper way" is usually for the employer to add a bonus to your W-2. There appears to be some legality about the organizational structure that the "management company" is not your employer.
This can be simplified.
Although it may be news to you that you “have a business” (and most of the other people who get their first 1099-NEC); The IRS considers anything on a form 1099-NEC to be self employment income. Enter the 1099 income, at 1099-MISC and Other Common income / income from a 1099-NEC . TurboTax (TT) will complete Schedule C for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & Medicare) on any profits greater than $432. These types of payments are sometimes done this way. Your "employer" is not the only one.
