1) To make an entry at Box 12 on Page 2 of the SA101, you need to add the Page - 'Other Income and Losses'. You can add this page by clicking the + NEW PAGE button on the left of the screen.
2) Once you have done that, you need to add a Group under 'Expenses' for Seafarers and Patent Reliefs.
3) You then need to add a Category for 'Foreign Seafarers Deduction' and make an entry under ENTER THE VALUE OF FOREIGN EARNINGS NOT TAXABLE IN THE UK
This entry should then appear at Box 12 on Page 2 of the SA101.
If you have paid the subscription, you can download a PDF of the SA100 at the bottom of the Report Page and see the entry you have made.