Hi @notabot 
The “Install button URL” input within your HubSpot App Marketplace Listing configuration generally should point to a webpage managed by the app. On this webpage, you could outline installation instructions (including any prerequesites etc.) and your HubSpot App Install URL. Based on my experience, the HubSpot App Marketplace Listing configuration doesn’t allow you to use your install URL directly within this “Install button URL” input (at the moment, doing this raises a validation error).
I hope that proves helpful. Please let me know if you have any follow-up questions.