SharePoint Online Custom Template ID

Hi,

How to find the TemplateID for a custom document library template in Sharepoint Online.

If I view the source code, the TemplateId is the Id of the original webpart used to create the template,

var g_wsaLCID = 1033;
var g_wsaSiteTemplateId = 'STS#2';
var g_wsaListTemplateId = null;