Pointing to a subdomain of a WP Multisite in App Engine

[AlexGSI]

I am trying to create a subdomain of a subdomain on my Multisite so I can Operate APP Engine in Google Cloud. I cannot create a subdomain of a subdomain vai Wordpress, so what is the best way to approach this? 

If I make a custom domain such as tracking.subdomainname.com (which is really mainsite.com/subdomainname ---displays as subdomainname.com), the "Google Managed, auto-renewing" under SSL security spins but eventually comes out with an error. Should I be adding something besides the 4 A records and 4 AAAA records to my DNS? I was told to not add the www cname as it would crash my site (it actually wouldn't let me add a www).