On the screen entitled "OK, let's get this info from your 1099-SA", the Payer is the HSA plan administrator.
This is generally the bank or financial institution that gave you a debit card with which to pay for qualified medical expenses. This information is located in the upper left-hand box of the 1099-SA under "Trustee/Payer".
Enter the Payer's name and address and federal identification number from the 1099-SA so that the IRA can cross-reference your 1099-SA with the copy that the IRS received from the HSA plan administrator.
Note that the next screen asks you to confirm your identity.