how to add a custom domain to long live channelid other than live?

### The use case you're trying to solve
as it's mentioned in the doc. ChannelID has one use case,

> You usually want to leave this blank so that each PR gets its own preview channel. An exception might be that you always want to deploy a certain branch to a long-lived preview channel (for example, you may want to deploy every commit from your next branch to a preprod preview channel).

I want to have a preprod.example.com pointing to the particular preview URL. which current contains a random hash.

### Change to the action that would solve that use case

I can set a custom domain either in firebase console, or via yaml file if I have *.example.com verifed.

### Other considerations