Form to provide for renting at less than fair market value?

While filling out taxes last year (for 2018), I recall seeing in the RENTALS section of TurboTax a way to determine the Fair Market Value of rent in my area. (I do not see that this year.) There was also mention that if renting to someone at LESS than Fair Market Value, there was an IRS form to give to them for that "gifted amount of rent" (the difference between Fair Market Value and what they were actually being charged). I cannot find mention of that anywhere this year - in TurboTax, the forum, or on the IRS website. Is this not a thing anymore? If it is, where do I find it?!