Adding this here for community feedback:
https://jira.camunda.com/browse/CAM-12740
Currently Forms on Start Events appear to only be supported on Blank Start Events.
While Forms traditionally have a UI component (the HTML of the form), the Forms are/can also be used as a API validation mechanism (such as a API call starting the process, triggered by bot, etc)
Only 1 blank start event is supported on a BPMN. But multiple Message Start Events are supported.
It would be very useful to introduce Form support on Message Start Events, allowing Message bodies (The variables being submitted as part of the message) to be validated against the form.
You can see previously conversations have occurred: Retrieve form key of message start event - #6 by StephenOTT