My goal is to set a Person or Group field value on a new record to a user who may not have logged into the subsite "mysite" (and does not exist at endpoint #2 below), but who does exist at root endpoint, #1 below.
For example, John Smith exists with Id
/sites/_api/web/siteusers, but he does not exist at all at
/sites/mysite/_api/web/siteusers. I want to store John Smith in a Person or Group field in a list located in
/mysite. John Smith has no Id for that site, however, hence the question.
I'm working with the SharePoint Online REST API using endpoints similar to these:
I don't think it is relevant, but the context is a custom web part built with the SharePoint Framework. The web part will be hosted on a modern page within /sites/mysite and is being tested on the tenant workbench (not local) with support files being served from my local machine.