The current app id is "org.bitcoinabc.cashtab"
The standard is for the app id to be reverse URL. Since cashtab lives at www.cashtab.com, a normal ID here could be com.cashtab.www
However looking at other crypto apps, it seems standard to use app instead of wwww (e.g. phantom does this)
I think tying to the cashtab domain for the cashtab app is appropriate, as the app will always have some kind of presence at cashtab.com