How would a WW1 pilot signal a request to surrender in-flight?

They Didn't

Even in in the modern era "surrender" of a combat aircraft in combat isn't a thing. There's just no way to do it (or accept it) safely enough. At best there are accounts of pilots defecting with their aircraft. But that's not really the same thing.

The closest thing to surrender a pilot/aircrew can do is bail out of their machine. The pilot/aircrew are intentionally making it so they are no longer capable of aggressive action. While that in itself isn't a formal surrender, the following rules apply Per Wikipedia:

"In 1977, this practice was finally codified in Protocol I in addition to the 1949 Geneva Conventions:[1]

Article 42 – Occupants of aircraft

1. No person parachuting from an aircraft in distress shall be made the object of attack during his descent.
2. Upon reaching the ground in territory controlled by an adverse Party, a person who has parachuted from an aircraft in distress shall be given an opportunity to surrender before being made the object of attack, unless it is apparent that he is engaging in a hostile act.
3. Airborne troops are not protected by this Article"

This developed after WWI when aircraft became powerful enough for parachutes to be common issue, and from a pilot's perspective a guy who bails out of a perfectly fine aircraft is basically surrendering. But that didn't happen in WWI.

In WWI for a variety of practical reasons (most pilots would rather have more ammo/fuel than a parachute of dubious value) and some official ones (the brits for instance forbade the use of parachutes because they felt it might make pilots bail from otherwise save-able machines) most pilots didn't use parachutes. They were the domain of the observation balloonists, whom pilots as a rule did not like (and frequently shot even when the balloonists were using parachutes!).

With no way to communicate a clear intention to surrender nobody tried it. Remember surrender is a dangerous situation, both for the surrendered (who needs must put themselves in the power of people trying to kill them) and the person taking the surrender (who must worry about it being a trick or exposing himself to the danger in other ways to accept the surrender). People in general would rather run than surrender, and the more dangerous surrender seems, the more likely they are to run. Even in situations like aerial combat where you might literally be incapable of escaping due to flying an inferior machine.

Surrender in air combat is meaningless - as there is no means to yield possession of the plane or offer one's person up as a captive. Instead, the concept of interest is escape. One might attempt this either powered or unpowered.

If a powered escape was attempted, it is likely that a pursuit would occur. This was one of the Red Baron's favourite ruses, with himself as the bait. He was shot down and killed, most likely by ground fire despite other claims, during one such attempt when he pulled out of his dive too low to the ground.

An unpowered dive involved turning off the engine and feigning an engine conkout. Pursuit might still occur, but not as reliably. Then one would restore power and pull out of the dive at a low altitude and return to base. This is a mandatory maneuver for all pilots to perform as part of obtaining their Pilot's Licence, and is quite safe provided the engine has not been damaged.

If one has performed a long unpowered dive it would usually be pointless to attempt return to the dogfight - the time necessary to return to altitude ensured that the dogfight was likely over, and most certainly would have relocated at least.