Go to GTM dashboard -> On the left sidebar menu -> Click on Tags -> New
Name the Tag such as Leadspace SmartForms tag 1>Click on the Tag Configuration box> side bar will them open where you will choose Custom HTML
Place script exactly as it is in .txt document sent with these instructions, will look like the example below, please ensure you use the HTML from your .txt file sent to you.
Check Support document.write
Use Page View
Add specific URL’s with contains
Or use All Page View
Publish all changes