All you have to do is copy the code your tool provides and paste it into the "Custom JS" box on your form. To find the "Custom JS" box, edit your form and click on the "Settings" tab. At the bottom of the settings options, you'll find it.
The script you paste in this box will be added to the <HEAD> of your form's webpage.
📝 Additional insights:
The body part of the tag manager doesn't have any effect on how it works.
You can either paste it in the script section of your formaloo editor or even ignore it.
Then test and preview your tag in the tag manager to make sure it works properly.