Referential integrity should not force a value to be in the Canvasser_ID foreign key field
Correct, but if you try to add a value to this field later and the value doesn't exist on the 1 side, you will generate an error. Perhaps the value OP tried to enter doesn't exist on the 1 side. That would be a good reason not to have a free-form text box for entering data in this field if that's the case (in lieu of a combo box).
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.