What exactly do you want to enter on Schedule 3 line 13z? There is no way to make free-form entries on that line. The only entries that are allowed on line 13z are a credit for tax paid to the Virgin Islands from Form 8689 or a credit from an excess limitation account under IRC Section 960(c). To enter one of these credits open the "Form 1040 or Form 1040-SR Worksheet" in forms mode. Scroll down almost to the bottom of the worksheet and look at the "Other Credits and Payments Smart Worksheet between lines 12 and 13 in the Schedule 3 section. Enter the amount of the credit on the appropriate line of the smart worksheet: line C for tax paid to the Virgin Islands from Form 8689 or line F for a Section 960(c) credit. The credit will then appear on line 13z.
