Can I add Tags based on form fills?

Yes. Tags can be added by form fills.

Follow the steps given below to add tags to contacts based on form fills:
1. Go to the Settings tab by clicking on the gear icon located on the top right corner.
2. On the left pane, click on Tagging Rules.
3. Click on Add New Rule button on the top right corner.
4. Select Based on form fill option in the popup that opens.
5. Choose the form name.
6. Select the Tag that you want to assign to contacts that fill the selected form.
7. Click on Add.