What you will need is the following on top of the default plane parameters with master:
SERVO1_FUNCTION=74 # right motor
SERVO2_FUNCTION=73 # left motor
obviously you should adjust trims as per usual. Note that the elevon trims are pre-mixer, so SERVO5_TRIM is your roll trim, not your left elevon trim (yes, I know this is weird ....)
then you will need ELEVON_OUTPUT set to the right value. Start with ELEVON_OUTPUT=1 and try all 8 possible values (1 to 8) until you get the right pitch/roll controls in manual mode.
Then set Q_TAILSIT_INPUT to what control system you expect to use. I think PX4 does body-frame controls? So aileron stick when hovering does earth-frame yaw (I haven't flown PX4 tailsitter, but I have heard this is what they do). If you like that scheme then set Q_TAILSIT_INPUT=1. If instead you prefer it to be like a multicopter, where aileron stick does earth-frame roll then set Q_TAILSIT_INPUT=0
PS: corrected for elevons being on 5/6, I misread it as 3/4 at first