Add a modern page with a modern template in SharePoint.

It’s easy to make a modern template in SharePoint now. You build your regular “modern page”, with your content type and after saving you click “promote”. A menu on the right opens and shows some sharing stuff.

You can save the page a s a “page template”. SharePoints saves it at “SitePages” in the folder “Templates”.

If you want to create a new page based on that saved template from a SP widget, use the file REST API:

/_api/web/getfilebyserverrelativeurl(‘${source_url}’)/copyto(strnewurl=’${target_url}’,boverwrite=false)

Source URL is the relative URL form your template:

const source_url = ${_spPageContextInfo.webServerRelativeUrl}/SitePages/Templates/Your-Template.aspx;

Attention: _spPageContextInfo.webServerRelativeUrl is only available in classic context. You need to find the webserverrelativeurl in the context object in a SPFx webpart.