Start with the example that has all the UI elements in it.
Then make a dialogue box ("panel") that has what you need (the drop-down, etc)
Include something like a select input, so the user can click on where they want the beam (hint: if it's a line, that also tells you how long)
Finally, when they click OK, it will run your command, so to grab the data they selected, and run your code for the dropdown they picked.
