Currently there is no known or documented way to enable a Mailbox Archive on EXO / MSO using a Restfull API.
The Exchange Powershell provides a way to enable but with MFA there is no way to do that by a service.
We can currently automate any User maintenance task using MS Graph API, but the Mailbox archive mode.
Wouldn't it be nice if we (all humans) could do everything with MSGraph?
Regards
Hannes