Failed to add custom domain to SWA with error message: Name <var> is not valid.

**Describe the bug**
CNAME record is generated in Azure DNS while an error message is displayed in the portal.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to Portal
2. Click on Custom Domains ->Add
3. Enter subdomain
4. See error
5. Navigate to Azure DNS
6. See CNAME Record

**Expected behavior**
A clear and concise description of what you expected to happen.
Not create the CNAME record and clearly describe the error.

**Screenshots**
If applicable, add screenshots to help explain your problem.

Adding the custom domain requires the FQDN and errors if the subdomain is provided.
However, the CNAME record is generated if using Azure DNS.

**Behavior**
Adding the FQDN resolves the issue.

**Additional context**
Add any other context about the problem here.