The problem with the Deeplinking to chat option is that you are redirected to the teams website then prompted to launch MS Teams.
I have been able to solve that by utilizing a combonation Deeplinking and the MSTeams protocol. MSTeams protocol works like SIP would for Skype.
<a href="MSTeams:/l/chat/0/0?users=user1@domain.com,user2@domain.com">Teams Chat</a>
Now Teams launches without the redirect to the Microsoft website.
