How to solve grant_type error with Rest web Services Connector

I set up a connection to a rest api. I selected the 'OAuth2 Client Credentials grant' and entered my credentials. When I test the connection I get an error: 'Connection test failed: OAuth Token request failed: {"error":"invalid_request","error_description":"The grant type was not specified in the request"}'. client_grant is a supported grant_type for this endpoint.

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