Adding user_id to OAuth 2.0 Authorization Client Credentials request body

I'm working with an API that is requiring user_id be passed in the body of the authorization (token) request.   I cant figure out how to configure the user id value in the authorization settings.  Without this value I'm getting an Unauthorized Client error message.   Can anyone tell me where I can add user_id to the Authorization body request with SoapUI?

Example:

x-www-form-urlencoded: scope: <scope>

grant_type: client_credentials

client_id: <client_id>

client_secret: <client_secret>

user_id: <user_id>

scope=read&grant_type=client_credentials&user_id=<user_id>&client_id=<client_id>&client_ secret=<client_secret>