Hi there, Fireline.
I want to make sure you're able to use the app you mentioned in QuickBooks Online.
In this instance, I recommend reaching out to a member of the Intuit Developer Support Team. An agent will be able to give you detailed info on what to enter for the app URL settings. You can get in touch with the team through the Developer Help Site by choosing Contact Support at the bottom of the page.
You're always welcome to check back on this thread later to see what other QuickBooks users have done in similar situations. That's one of the great things about the Community.
Please feel free to reach back out in the comments below if you have additional questions. I'll be here to help in any way that I can.