begin
Client.BaseURL := 'APIURL';
Client.Params.Clear;
Request.Params.Clear;
Request.Method := TRESTRequestMethod.rmPOST;
Request.Params.AddItem('data',JSON,TRESTRequestParameterKind.pkREQUESTBODY);
Request.Params[0].ContentType := DefaultRESTContentType.ctAPPLICATION_JSON; // important
Request.Execute;
end;