The $1,000 "catch-up" contribution actually refers to the increase in the annual HSA contribution limit when the HSA owner is 55 or older.
TurboTax automatically adds this increase in the limit for any HSA owner who is 55+.
In your case, you want to make a "personal" or direct contribution to your HSA. You do that by (1) sending a check directly to your HSA custodian before April 15, 2019, (2) making it clear that this is a contribution for 2018 NOT 2019, and (3) going to the "Let's enter [name]'s HSA contributions" screen in the HSA interview, and entering (or adding) this amount to the second line, which reads "personal" contributions (i.e., not through the employer).
You don't need to have any external form like a 1099-SA to do this. In fact, the 1099-SA reports distributions from the HSA, not contributions to the HSA.
NOTE: you must have space on your 2018 contribution limit to add this $1,000 amount, that is, the sum of what you already contributed plus this $1,000 must be less than or equal to your calculated annual HSA contribution limit for 2018...and, of course, you must have been 55 or older by the end of 2018.
