Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you would like to configure a CNAME for your Azure VM.
Please note that to configure a CNAME for a domain, you must own it.
In your case, the domain is "southeastasia.cloudapp.azure.com" - which is of course not owned by you.
Also, in this case, you should configure your Mobile App to use Traffic Manager's Domain, and not the VM's (hostname.southeastasia.cloudapp.azure.com).
Refer : How Traffic Manager Works
FIRST - You may own, "contoso.com"
- If you do, you can add the Traffic Manager's domain as a CNAME to <yourappname>.contoso.com
- If you don't , you can directly use the Traffic Manager's domain as DNS for the VM
- So when someone queries for <yourappname>.contoso.com , they will get <example>.trafficmanager.net
SECOND - Traffic Manger's domain,
- Now, the traffic Manger's domain will be queried , i.e, <example>.trafficmanager.net
- Then, <example>.trafficmanager.net will give your VM's domain (based on the routing method you have specified)
- https://learn.microsoft.com/en-us/azure/traffic-manager/traffic-manager-routing-methods
THIRD - VM's domain,
- Now, your mobile app will get the VM's domain and it can resolve it to the VM's public IP.
Please let us know if we can be of any further assistance here.
Thanks,
Kapil