Hi
Foreign trust income is declared on a paper tax return SA1107, along with a paper SA100 or if you have a government gateway user ID and password, you can choose between submitting a paper tax return and buying a commercial tax return that supports trust income.
You cannot use HMRC online tax return, as it does not support trust income.
You can find a list of commercial suppliers at
Self Assessment commercial software suppliers.
You ar required to submit the tax return in its entirety in the same format.