Getting an Oauth Token

I am trying to get an oauth token by posting to the following but getting a "Permission Denied" message for the credentials that I have supplied.  The account that I am passing has been assigned the REST Web Service Role.  Are there other roles it would need for getting an oauth token?

https://illumina.na5.xmatters.com/api/xm/1/oauth2/token?grant_type=password&client_id=7469ebe0-4dff-4a1b-84fe-0d1b3baf9dcf&username=username&password=password

Note: The above URL is an example of the call I am making.  For the actual call, I have substituted my Client ID and the user name and password for the account in my tenant that has the API role.  Still permission denied...