### Steps to reproduce

Begin of the deletion _Please write down the steps to reproduce. Try to write down all necessary preconditions (what permissions do you have, are other users involved?). Example:_

End of the deletion 1. Either look into the API yml files or the openapi json / Begin of the insertionyaml.

End of the insertion Begin of the deletion yaml. 
End of the deletion 2. /memberships POST has no parameters Begin of the insertion


End of the insertion Begin of the deletion

End of the deletion ### What is the buggy behavior?

Begin of the insertionYouEnd of the insertion Begin of the deletion _YouEnd of the deletion are unable to create memberships via API, because the API schema has no parameters on /memberships Begin of the insertionPOST. End of the insertion Begin of the deletion POST._ End of the deletion
Begin of the insertionIfEnd of the insertion Begin of the deletion _IfEnd of the deletion you use openapi-generator, creating memberships via API is not possible due to missing parameters, so createMembership() has no params as well. The only way is to use the raw API client and blindly post a JSON to create a Begin of the insertionmembership.End of the insertion Begin of the deletion membership._End of the deletion

Begin of the insertionSeeEnd of the insertion Begin of the deletion _SeeEnd of the deletion also Begin of the insertionhttps://community.openproject.org/topics/16124?r=16124End of the insertion Begin of the deletion https://community.openproject.org/topics/16124?r=16124_End of the deletion

### What is the expected behavior?

1. Define the correct schema to create memberships in the API schema Begin of the insertion


End of the insertion Begin of the deletion

###

End of the deletion Even the latest dev version has no parameter on POST. See [https://github.com/opf/openproject/blob/dev/docs/api/apiv3/paths/memberships.yml#L126](https://github.com/opf/openproject/blob/dev/docs/api/apiv3/paths/memberships.yml#L126) Begin of the insertion

End of the insertion Begin of the deletion


End of the deletion Affected are 12.x, 13.x and dev. Didn't check older versions.