So are you saying that you want to re-select one sub-form based on the (manual) selection of the other subform? What makes it unclear is this:
From post #1 of this thread:
populating a field on a sub form based on another field on the same sub form
But later in post #4 you said:
I want to populate the subsccountID field based on the selection from the AccountID combobox. Both comboboxes are on the subform.
So now it is not merely a field, it is now two combo boxes, one with a manual selection and the other with an automatic selection. Is this a reciprocal situation where you could have made a choice on the 2nd box to reset the selection on the 1st box? Or is it one-way only?
I have no doubt we can get an answer, but first we need a consistent question. We understand that new members have trouble with questions sometimes, which perhaps is why Eugene responded as he did.
I'm going to take a stab at this based on guessing what you want. Here is a link with some code.
I have Access 2010 form which has a ComboBox cmbSubTopic which lists two columns (SubTopicID and SubTopic). The combo box is bound to a field containing SubTopicID. The SubTopicID column in the com...
stackoverflow.com
The idea is that you must find the combo box row that has the value you want and set that row's .Select property to true. You would have to previously do an .Undo on the combo if it already has a selection. You can tell if a combo has a selection because the cbo.ListIndex will be something other than -1 when a selection has occurred. (Note that the combo's list index starts at 0, not at 1.)
You DO NOT jam-cram a value into the box because alters the underlying rows in some cases. You have to FIND the row and then select it.
Maybe that link and my explanation will be enough to help you. If it is not, then you must consider trying to improve your question. But maybe I've provided what you really wanted here. And if I'm close, at least you can see the general principle involved.