a-tisket does link to another tool that can submit ISRCs as part of the workflow, the tool being:
I haven’t used the tool independently from a-tisket, but it looks like you can if you choose. it seems to only submit from Spotify though…
edit to actually comment on the release:
I will note that some digital releases do have catalog numbers, it’s just that most stores don’t display and/or store that information (an example from Japanese site OTOTOY). I usually leave that field blank, or sometimes [none] if it’s self-released (like a lot of Bandcamp releases)
I also see someone has added the long countries list of release events. you don’t have to do this, but it will be added by default if you use a-tisket.