Technically, yes, you could allow it even without https inspection enabled. That blade is never needed to add custom app site, as long as you have URLF blade enabled in the gateway, works fine. Inspection is more if you want firewall to intercept the traffic and "insert" its own cert that would get presented when pages are blocked and it makes sense to have it, since probably 99% of sites now days are indeed https.
I made this work in R81.10 and R81.20 lab just fine without https inspection on. Happy to do remote if you need help.