Set text box equal to variable

[Gavx]

Not sure whether this should be in this or the Forms forum.

I have a form containing a sub form.

The subform contains several fields of a record, but not all of them.

When the user selects a particular field of a record on this sub form a button becomes visible, prompting the user to select, which would then open another form.
I want this button, linked to some code (if that is the best way) to capture a related field of this record and record it in a text box located on this form.

What would this code look like?

thnks
g

 

[Ranman256]

assuming the subform box is called child1:

txtBox = child1!form!txtField

(use the BUILDER, it will let you pick and choose the box names)

 

[Gavx]

Using Builder I get #Name?
Builder has written the formula as

=[Forms]![frmGuestsbf]![txtGuestID]