Hi David,
Firstly: I assume you’re talking about a persistent field, not a read-only custom field.
There are limitations on what you can do in screens like this (assistants). The assistant creates something like a purchase requisition (LU = PurchaseRequisition).
While it might be possible to add a custom field to that resultant LU, and it might be possible to add that custom field to the Assistant, so that it shows up on the page, the framework does not yet exist that will receive the value you put into the field and populate it into the resulting purchase requisition.
This sounds weird, but it makes sense if you imagine that the assistant is assembling data which is then passed into the MakeRequisition logic. The MakeRequisition logic doesn’t know what to do with the custom field value.
This might be improved later-
But this has been my finding up to now.