How to grant an active directory group permission to a sharepoint online library using powershell?

Here's what I'm trying to do with powershell/sharepoint online management shell:

Grant an Active Directory group: "group 1"
a custom permission I created in SPO "custom permission"
to a library "library 1"
in my site collection "site1"

I can't figure it out, but I'm sure it's possible. I found some articles detailing processes that would effectively allow me to do this, but involved adding an AD group to SPO group, which would necessitate creating a SPO group for every library.

Any ideas?