Leaf.xnet can't send post data

[1.538]

I get 403 error, I am sure because of the post data.
The post data looks like this: grant_type=client_credentials
tried:
var res = request.Post(url, "{grant_type=client_credentials}", "application/x-www-form-urlencoded");

var res = request.Post(url, grant_type=client_credentials, "application/x-www-form-urlencoded");

var reqParams = new RequestParams();
reqParams["grant_type"] = "client_credentials";
var res = request.Post(url, reqParams, "application/x-www-form-urlencoded");

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Edit: It was a fucking letterspace in the authentication code