@ljutc70 Julian is correct - the payload feature was added in main, and will be present in v1.14.
In PX4 v1.13 your only choice was to use MAV_CMD_DO_SET_ACTUATOR - and you might have to add it to the mission manually and import that to get it into QGC. The process works but the mission has no awareness of the payload - for a gripper, for example, it might just trigger, start releasing the payload, and then fly off.
So if you use this approach you may have to add delays etc in your mission to make it work.
Docs on the actuator setup are here: Payloads and Cameras | PX4 User Guide
PX4 v1.14 is supposed to be just around the corner, so you might want to start flying that instead.