One text box populate another text box?

[Xterra14s]

Hi, so I have one text box with a patients ID number that I need to update to another text box in a subform with the same number. I did get it to work but that was by having the text box unbound. I need it to be bound to a field that will store data into the table. Is there a way to do both? Have it auto populate based off of another text box but still be bound to the patient ID in a table? I'm assuming it involves VBA.

[ranman256]

make both boxes bound.
the sub form MUST connect to a table/query.
the sub form MUST be linked via the ID box via properties:
LINK MASTER FIELD
LINK CHILD FIELD

once linked the subform ID will autopopulate when you enter data.