You could use DLookup() in some code. Have you tried this yet.
Alternatively, perhaps you could use a combo box to enter the idnumber. If the combo is populated by the data in the "other" table then you could populate the text box from a hidden column in the combo.
Cross posted: https://www.accessforums.net/showthread.php?t=81807
It has already been suggested to you that you have a problem with your table design and that until that is sorted out you will have problems.