I'm not sure why you would want this. Do you have more details? What are you trying to do, with the form ID? There's probably a better way to get there.

In general, a Layout Template is used across multiple forms.

So if you have multiple <iframe> forms embedded on your website, you assign them all the same Layout Template. Now all the forms look the same (fonts, colors, buttons, etc.). If you need to change the button color (from red to blue, for example) you would make that CSS change in one place - that Layout Template shared by all forms - and the change happens across all the forms at once.

So your question seems like it ties a single Pardot form to a single Pardot Layout Template, which would mean you need different Layout Templates for each form ... making it hard to do those kind of mass updates.

So that's why I'm wondering what you're trying to do, so there can be a more efficient answer.