Okay. We found the Soution.
What you need to do is creating an Alternative Description where you use HTML.
Just add following snippet to your iCal String:
X-ALT-DESC;FMTTYPE=text/html:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><HTML><BODY><a href="msteams:/l/chat/0/0?users=EMAIL" target="_blank">LinkText</a></BODY></HTML>