Sorry, I did not mean to include the [Record ID#]
I think that the problem is where the related parent field is set to read only on the form which is annoying that it does not populate.
There are other option if you are unwilling to make that field be read only. One way might one to have a form rule to say,
when related parent is not blank, then make read only. Then the field would not be read only until it gets populated.