If you don't have single sign on (SSO) or automatic logon enabled, you could change the graphic above the user/password fields on Tableau Server's sign-in page to include some consent text like "by logging in, you agree to...". There won't be a checkbox for them to click, however.
Hope it helps! - Kye
I haven't personally done this, but heard someone else did the following:
Customize the tableau server login.html to include a link that asks new users to click on a link to fill a confidentiality agreement. When the user signs the agreement, their userid is added to a AD group. They used REDCap server to collect user registration requests and then processed the requests using an automated alteryx workflow which makes REST API calls to the REDCap server and Tableau server. Once they're added to the AD group, they can then login using SSO to Tableau Server. This is the only tips I can provide as I haven't done this before.
Hope this helps, Tony