My Business Account registration for non resident corporations

Hello all,

We have a new client, non-resident corporation.
The owner of the corporation applied for an ITN number so he can register for his own “My Business Account” and add us (their accountant) as representatives.
The owner received the ITN in the mail, as expected.
He is trying to register for My BA, but the third question is… “Enter line 15000 from oyour 2023 income tax…”.
How does he proceed with this question if he never filed a tax return?
Thank you.