The heading field on a waypoint is only show for firmware/vehicle type combinations which support it. That said it tends to not work even then because there is a parameter which locks heading to point towards the next waypoint. There are other mission commands I think which can be used to do what you want. I'd ask somewhere on a firmware forum, describing what you want to do and what command to use.
The mavlink docs (https://mavlink.io/en/messages/common.html) specify what ever parameter in a mission command means.