Unfortunately it does not seem this is possible.
this is from another thread i found here: https://serverfault.com/questions/997025/hostname-not-eligible-for-app-service-managed-certificates-creation
App Service Managed Certificates can only be used with URL's that are setup as DNS CNAME records. "www.stratus-integration.com" has been setup as an A record, not CNAME. You will need to delete the A record and create a CNAME record pointing to the app service DNS name.
Is there a possibility you can create the cert elsewhere and just use a regular SSL Cert?
HTH