I was a shareholder/employee of a business that went bankrupt. I had to pay money as a guarantor of a loan to the business. How do I deduct that payment?

[RobertB4444]

Your 'basis' in the business is the total amount that you had invested.  That includes the amount you initially sent in as well as any payments that you made to keep the business afloat (or close it) minus the amounts that were given to you over its life.  

When your investment becomes worthless you can take a capital loss for the basis that you had invested at the time.  In the wages and income section of your federal return you'll scroll down to the 'Investment Income' section and click start next to 'Stocks, Cryptocurrency, Mutual Funds, Bonds, Other'.  (you're the Other)

You'll enter it as a sale of the investment.  You'll enter the purchase date as the first day that you invested in the business and the basis as the amount you already figured out.  The sale date is the date that the investment became worthless and the sale price is zero.

@dpybus1