You can add Owner role a subscription from its main blade in the Azure portal.
- Select Subscriptions in the navigation bar on the left.
- Select the name of the Subscription.
- Select Access control (IAM)from the left menu.
- Select Add on the Access control blade.
- Select the Owner role that you wish to assign from the Select a role blade.
- Select the user that you wish to grant access to. You can search the directory with display names, email addresses, and object identifiers.
- Click Save button.
Note: Classic subscription admins and co-admins are considered owners of the subscription in the new RBAC model.
After successfully adding a role assignment, it will appear on the Users blade.