AMT taxable income from form 6251 line 4. I don't have that form. Help text vaguely says you can calculate with 1040 line 15 + line 12. Is that the amount I enter or ?

I added carry over capital loss from 2023 and this AMT step is confusing in Turbo Tax. Since I don't have form 6251 from 2023 do I calculate the value for this field or leave it blank?