“Take the SharePoint document library id from the url“ is the most common answer, but from where exactly and how?”
Go to Library settings and copy/paste the url from browser into a text file. It is in the url behind the List= starting with %7B and ending with %7D, here’s an example:
https://<my>.sharepoint.com/sites/<my_site>/_layouts/15/listedit.aspx?List=%7B1ff350c6-80b7-4454-bfa7-5cb662665c64%7D
Remove the %7B (ASCII code for character { )and %7D (ASCII code for character } ) and you get the library ID, in the example above the library ID is:
1ff350c6-80b7-4454-bfa7-5cb662665c64