Cause
The body includes a field the API never accepts: company, authorship, environment, ownership, privileges, permissions or grant selectors. details.fields lists up to five. It is rejected instead of ignored.
What to do
Remove the listed fields. The company, the integration and the environment come from the credential.
application/problem+json
{
"type": "https://masnet.com/developers/errors/forbidden_field",
"title": "Forbidden field",
"status": 400,
"detail": "...",
"code": "forbidden_field",
"correlation_id": "..."
}