Hi @ExtraLargeFork
You can use this link to navigate to a chat with a user:
https://teams.microsoft.com/l/chat/0/0?tenantId=<tenantId>&users=<user1>,<user2>,...&topicName=<chat name>&message=<precanned text>
If you don't have an existing chat with the user, this link will create a new chat window. If you already have an existing chat with the user, this link will directly navigate to the existing chat window.
Here is an example for your reference:
https://teams.microsoft.com/l/chat/0/0?users=joe@contoso.com,bob@contoso.com&topicName=Prep%20For%20Meeting%20Tomorrow&message=Hi%20folks%2C%20kicking%20off%20a%20chat%20about%20our%20meeting%20tomorrow
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