Is it possible to create the Use Template button on a google site?

@Mishal - I am 99.9% sure that you will not be able to invoke the Use Template call--that is already in a modal window--from an external source. You may be able to script this using custom JavaScript that will work from you, from your browser, but will not work for anyone else.

Luckily because you are looking to share Custom Templates, rather than storing you templates in the “Custom Template” editor/framework, you could manage your Custom Templates as private boards what are in Anyone with the link → Can view mode and the Board Content Settings set to allow for anyone  with the board access to be able to copy.

Here’s how the process could work:

1. Copy your Custom Template to a new board
2. Create a folder called “Templates (Public)” - or something similar - basically “Templates” first, to keep things alphabetical should you want a “Templates (Private)” or “Templates (Draft) folders.
3. Set the board’s Share settings to
   1. Team access → No access
   2. Anyone with the link → Can view (or set to “No access” until you are ready to share)
   3. Sharing settings → Permissions → Who can copy board content → Anyone with the board access (if you don’t see this option, a Team Admin will need to configure it)
4. When the Template is ready for consumption, you would share a link to it anyway you’d like with the instructions that the user
   1. Clicks on the board title
   2. Uses the Duplicate action

Try it now with this board that I have configured using the above steps:

https://miro.com/app/board/o9J_kl4Sue4=/