> If I have a default value, with a unique index on this field,

Unique indexes are usually considered bad things, since they don't reflect
what's in the table. Candidate indexes, otoh, restrict you from entering
duplicate values.

Also, if this is the field that relates the two tables, the user should
not have control over it. It should be a surrogate key, with no meaning
beyond relating the tables.

If your concern is only not saving a blank record, you can handle that
with a record-level rule, which will fire when the user tries to click
onto another record, if you are using record buffering.