If understanding right you are facing trust issue while using App service url.
Suggest you to add a custom domain binding for that URL and configure an appropriate PFX certificate against it for the site to start responding on the URL.
For production applications, we highly recommend that you configure a user-friendly custom domain and protect it with SSL via a PFX certificate. The certificate can either be purchased within Azure via App Service Certificate or from a Trusted Third-Party provider and uploaded to Azure.
Please refer to the following documents in case you need assistance configuring a custom hostname and associating a PFX certificate with it.
- Map an existing custom DNS name to Azure Web Apps
- Map a custom domain via App Service Domain to the website
After investigation its detected that, you are currently running the site on a Free SKU. please see the supported SKU's are as follows
- Custom domains : Shared SKU's and higher
- SNI SSL: Basic SKU's and higher
- IP SSL : Standard SKU's and higher
For details see- Azure App Service Plan Pricing Information
Let us know if further query or issue remains.