I have an existing custom sub-domain which I have mapped to my existing Azure App Service web app via CNAME. I have also assigned this custom sub-domain to my app service app via Azure portal successfully. I can see "HOSTNAMES ASSIGNED TO SITE" section as expected and also access my custom sub-domain to see default azure app service content page.
To buy SSL certificate I'm following steps on MS docs here.
I can't complete Step# 4 Verify Domain Ownership. I selected App Service Verification method to verify ownership. However, I continue to see following message -
No App service app configured with the custom domain : my-sub-domain