The easiest way to do that is to embed an SSO login link on your forum. You can get the format required for the link by viewing the page source code for the Login with Discourse link on your WordPress login page. You can probably embed the link without URL encoding the redirect parameter.
Something like this:
<a href="http://wpdiscourse.test/?discourse_sso=1&redirect_to=http://wpdiscourse.test/">Log in with Discourse</a>