A family of Microsoft relational database management systems designed for ease of use.
You are trying to copy the part number from the programs table to the table with the produced units. That is putting the same data in two places. The only thing that can appear in more than one table is the primary key from a parent table. A field in a child table that contains the primary key from another table is known as a foreign key field. The foreign key is the only information needed to retrieve all the other fields in the parent table's related record.
If your boss told you to do anything else, then you no longer have a relational database and might as well be using a spreadsheet.
Alright, lecture over. As reluctant as I am to facilitate your creation of an unnormalized problem database, I'll try one more time to explain how to replace a text box expression with VBA code so the calculated value can be saved.
If the [Part Number] text box uses the expression:
=Me.Combo69.Column(1)
and displays the desired value, then you can change the text box's control source from the expression to the PN field in the form's record source table and use the line of code:
Me.[Part Number] = Me.Combo69.Column(1)
to set the value. The text box should then display the same value as before AND save the value to the PN field in the form's record source table. If it does not, then you have missed something somewhere.