REF only goes to a bookmark. You can put a Content Control inside a Bookmark, but why bother. If you have a mapped control, simply put another copy of the control where you would use the REF field.

References and Add-Ins:

• Mapped Content Controls by Greg Maxey
• Content controls in Word | Microsoft Learn (This is a long article. Look at XML mapping toward the bottom.)
• Walkthrough: Bind content controls to custom XML parts - Visual Studio (Windows) | Microsoft Learn
• Insert Content Control Add-In by Graham Mayor
• Custom Content Control Insert Dialog by Greg Maxey
• CC⁄V⁄BM & DP Tools Add-in by Greg Maxey, MVP and Graham Mayor