Share OAuth2 access token between subdomains

I have a bunch of subdomains: auth.example.com, api.example.com, dashboard.example.com, and reports.example.com. The reports site, by default, displays some basic reports (i know, duh).

Now if a user logs in to the dashboard via the auth page (hosted on the dashboard and auth subdomains, respectively), i want to display a bit more data. The trick is that both the dashboard and the report site is a PSA that fetches data from the API. The report endpoint of the API optionally accepts an OAuth2 token and, if present and has the necessary access rights, will respond with the extended data set.

The question is, how should i share the access token? Storing the token in a non-HttpOnly cookie (i canʼt use a HttpOnly cookie since the data exchange happens in AJAX requests). Is there a better way?