Capturing OAuth2 Token in a Vugen Script

Hi Experts,

I want to capture a token authetincation (OAuth2 Token) and  after that I want to pass authentication to a REST_API call by a web_custom_request. 

 And the way I'm do it doesn't work, i tried to capture the authentication using below

Web_Set_User("UserName","Password");

Web_add_header("Client Identifier","Value");

Web_add_header("Client Secret","Value");

 web_custom_request("OAUTH2", 
        "URL=https://xxxxxxxxxxxxxxxxxxxxx", 
        "Method=POST",  
        "Resource=0", 
        "RecContentType=application/json", 
        "EncType=application/xml", 
        "Mode=HTML",       
        LAST);

I tested the same values in SOAPUI application and I can get a response back from authentication.

Please advise.

Thanks

Sathish Ramasubbu