How to link an external website to open up the 1on1 discourse message menu

Dear Discourse dev community: We have a website: Haddee.com, it has a single sign on feature working. What we want to do: when a use click another user and it will open up 1on1 discourse message feature. All of our users have discourse accounts.

How do we get this done? any guideline?

thank you!

Dandan
Founder, Haddee.com