It looks like this might be possible by specifying a custom machine image, per New feature in Juju 2.8: Add custom machine images with the juju metadata command it should be possible to tell juju to use a pro image with:
juju metadata add-image --series <series> <image-id>
Where series is {trusty,xenial,bionic,focal} and image-id would be the AMI of the Ubuntu Pro image.
The AMI for Ubuntu Pro 18.04 is ami-09109b16d2d9d5779, so you should in theory be able to use:
juju metadata add-image --series bionic ami-09109b16d2d9d5779
I haven’t tested this yet, but it seems possible. It might be necessary to first subscribe to Ubuntu Pro in AWS Marketplace, in order to be able to access the AMI.
It would be nice if Juju could make it easier to enable Ubuntu Pro on models in public clouds where Pro is available.
Update: It doesn’t seem to work 
I tried the above, obtaining the AMI ID from AWS marketplace two ways:
- Using the AMI displayed on the “Configure” page for the Pro listing
- Adding it to Service Catalog and copying the AMI out of the template for the product
In both cases, Juju can’t find the image:
Every 2.0s: juju status slate: Wed Jul 29 12:36:53 2020
Model Controller Cloud/Region Version SLA Timestamp
test aws-us-west-2 aws/us-west-2 2.8.1 unsupported 12:36:54-05:00
App Version Status Scale Charm Store Rev OS Notes
ubuntu waiting 0/1 ubuntu jujucharms 15 ubuntu
Unit Workload Agent Machine Public address Ports Message
ubuntu/0 waiting allocating 0 waiting for machine
Machine State DNS Inst id Series AZ Message
0 pending pending bionic failed to start machine 0 (cannot run instances: The image id '[ami-0fd6beacda77c37e2]' does not exist (InvalidAMIID.NotFound)), retrying in 10s (9 more attempts)
I’ll open a bug.
Update: LP#:1889458