If you simply need a button which takes you to the chat window in teams (web apps - provided that you are logged into teams), then you can just add a "Button" web part on your site page (modern page) and then bind the following link to the button click by editing the web part. Clicking on that button will open the teams web apps chat window.
As per your requirement, you can use the url in the below format:
where <user1> & <user2> are email addresses of the desired users.
To ask whether to open in app or web use the below format:
To force open the link in teams web app, use the below format:
For more information please check this documentation.