Finding the ID for you custom template can be hard if you don’t know how to do it.

For this step I prefer to use Google Chrome, Chrome is easier in finding code than IE.

Navigate true the Tenant admin to the site which you want to use as you custom template (when you have saved this this will appear in the TAB custom).

Example https://tenantname.sharepoint.com/_layouts/15/newsbweb.aspx

When you navigate to this URL you will see the section to select a template:

Now as this is open in google chrome right click on your template and click ‘Inspect Element’

You will need to expand the following code and press inspect element.

Once you have expanded the code you will receive the ID associated to the template.

So in my case ‘TestTemplate’ ID is “{6E9A9064-9174-4BEC-9A95-FEF99F7D1CFC}#TestTemplate ”

Keep in mind you must use the entire id including the name of the template itself and the “{ }”