Apologies for late response! If you want to deploy from a registry in a different subscription, select Private Registry in Registry source instead and then follow the next step depending on the Container Type:
For Private Registry:
In Server URL, type the URL of the server, beginning with https://.
In Login, and Password type your login credentials for your private registry.
For Single Container, supply the image and tag name in Full Image Name and Tag, separated by a : (for example, nginx:latest).
If you want, type the start up command in Startup File. App Service appends the string in Startup File to the end of the docker run command (as the [COMMAND] [ARG...] segment) when starting your container.
Let us know if you have further query or issue remains.