Did you try calling this REST-service without the Content-Lenght in your custom headers? It looks like custom-lenght is automatically added to the header already.
grant_type is related to Authentication, worth checking whether REST service need some authentication i.e basic or oAuth 2.0 etc.
HI Mikel Castro,
Is your API working well in postman, if yes, check the headers and the request that is sent from the postman and add the same in mendix.
You can trace the request in the postman console and check what all headers are required.