Additional (Easy Invitation Code) field on registration

you can use reference meta field during registration. Please see Notes tab for it.

Alternatively, you can also use update_user_meta endpoint after registration of user like this /api/user/update_user_meta/?user_id=234&meta_key=invite_code&meta_value=3473924732

Not working, it is possible to register without Easy Invitation Code.
On normal site registration form you need to enter invitation code, but using this plugin it is possible to register without invitation code.

of course, this invitation code plugin wont work out of the box for this API. you have to call and store it yourself in the api call. I just gave you an idea how it could be done.

Please check your user_meta with normal registration form this invite code plugin most probably saving meta_key value for that user with the invite code. you need to save the same meta_key variable with this api update_user_meta endpoint call.