You're missing a gating if for only running the code when it's being placed. If you don't have this code you'll get the prompts every time any stamp is selected.
You'll find everything you need to know about stamps here:
Here's an early article on creating advance dynamic stamps. It also includes the gating code you need, which relies on the stamp name, which is explained in the article:
In order to have multiple input fields (or check boxes, pulldowns, radio buttons, etc) entered at once in a custom dynamic stamp you need to use a custom dialog. Coding a custom dialog is an advanced task but there is an online dialog designer that makes the job much easier than hand coding- ACRODIALOGS OVERVIEW
There are examples of custom dialogs used with stamps in the video PDF Stamps Gone Wild so you can see how they work. That video can be found at-
Hope this helps,
Thank you Dimitri.
Thank you Thom.