private Optional<NameValuePair> resolveGrantType(final Service service) {
        return getRequestParameter(service, OAuth20Constants.GRANT_TYPE);
    }
This is {@link OAuth20AuthenticationServiceSelectionStrategy}.

@author Misagh Moayyed
@since 5.0.0
support/cas-server-support-oauth-core-api/src/main/java/org/apereo/cas/support/oauth/services/OAuth20AuthenticationServiceSelectionStrategy.java