Expression in textbox can reference combobox or listbox column by index. Index begins with 0. If value is in 2nd column its index is 1.
=[comboboxname].Column(1)
This will also work for a single-select listbox. If listbox is multi-select, VBA is required.
This will not save to table field. Saving will require code (macro or VBA). Saving this duplicates data between tables and usually is not appropriate.