You would use an Ask Question Action and an Update Ticket Tag action to apply the Tag. You would use Variables for the Update Ticket Tag action and use the Input Variable for the Tag Value so it prompts for input. That would look like this:
Actually just using Update Ticket Tag should work:
Update Ticket Tag Action:
Rule to handle it as a prompt:
This sets it so as soon as you create a ticket it asks for Guest Count. You can set this automation anywhere you like if you do not want it asking for every ticket. I used this as an example.
Remember you still need to define a Ticket Tag for it to show on reports I skipped this step as you already knew that from the tutorial. The tag i defined was Guest Count.