You can add a tag to your contacts when they click on a button with a link in your email campaign or automation email. In this help article, you can read how to set this up in the email designer and classic designer!
Before you can add a tag to a button with a link, you need to drag the 'Button' content block into your email. After clicking on the block, you can click on 'Add campaign action' in the menu on the right.
A pop-up screen will appear with the text 'Add an Action'. Here you can click on the drop-down menu 'Select an action'. Then choose 'Add a tag', so you can specify which tag(s) you want to add.
Please note: don't forget to actually add the URL / link as well!