Need Help: Calling OAuth 2.0 Token Endpoint from Oracle SOA BPEL – "Missing Required Property: grant

I'm trying to invoke an OAuth 2.0–protected REST API from an Oracle SOA Suite 12c BPEL process. The first step is to fetch the access token using the client_credentials grant type. However, I'm consistently getting the following error from the token endpoint:

{
"errors": [
{
"code": "OBJECT_REQUIRED",
"dataPath": "",
"message": "Missing required property: grant_type"
}
]
}

Endpoint: https://api.smartrecruiters.com/identity/oauth/token

Method: POST

Headers:
Content-Type: application/x-www-form-urlencoded

Request Body:grant_type=client_credentials&client_id=xxxxx&client_secret=yyyyy

Sent the request body as a string variable in BPEL

Mapped the payload to the input of a REST Adapter