Well, describing the problem more clearly would certainly help.
Typically a subform will have many records related to a single
mainform record. Do you want to use this unbound textbox as a link (if
so, you can use it as the Master Link Field property of the subform,
or as one of the fields in that property)? do you want to run an
update query updating all of the subform records with repeating values
drawn from the textbox? What's the context?
John W. Vinson[MVP]