There is no button for this on the controller.
You get to this function via the Ophelia-go app on your phone.
See page 32 '11.4 Headless Mode' in the manual.
Make sure that you remain facing in the same direction, or it can becomes very confusing.
A picture's worth a thousand words... Antenna wire is coaxial: a center conductor, insulated and shielded by braided grounding wire. Strip to the center conductor then remove another 1/8 to 1/4 inch of black insulation - "fold" the braided insulation back over the black insulation and use a needle point soldering iron/gun.
I believe you will need the Ophelia GO application. I'd advise against headless mode as it's only good (in my opinion) for direct line of sight, close in flight. Things get real confusing fast. Your mileage may vary. I've attached a scan of page 28 of the Instructions For Use booklet that came with my HS720