Forms :: Copy Value From Field In Subform To Field In Main Form During Data Entry

How can I get the value from a field in one table (in the sub form) to copy/insert into a field in another table (in the main form) when adding a new record?The main form and sub form are linked using parent/child linking, and the sub form is in a tab.I have table A (Visit Dates) in the main form which is used to record the date of a visit to a church. Table B (Quarters and Peals) is used to record an event that took place at that church during that visit. Note that not all visits in table A require a record to be created in table B - but half or more do.

In tables A and B I have a field called "QuarterOrPealID" and these are both primary keys, though the field in table B is set to 'no duplicates' and in table A it's set to 'duplicates allowed', as table A has its own auto number/pk. They are both linked in the relationships.

So, when I add a new record to table A using the main form, I might then need to click on the tab in the sub form to create a new record in table B, which has to be linked to the same record in table A. When the "QuarterOrPealID" auto number/pk is generated in the sub form (table B), I need that value to update to the "QuarterOrPealID" field of the main form (table A), so that when I'm viewing these records the form pulls all the information nicely together.