This can be made to work. You need to send the user to /session/sso?return_path=/new-topic?title=your topic title
.
A full link would look something like this:
<a href="https://forum.example.com/session/sso?return_path=/new-topic?title=this is another test topic">New Topic</a>
Have a look at the Create a DiscourseConnect login link topic for more details.