Great question. I believe what you’re referring to is the requirement for users to first install your app using the “Share your app” installation link under this section:
This is only a requirement for apps that have not yet been approved in our marketplace. This manual installation requirement on the part of the user account’s admin is just a security precaution, given the fact that the app has not yet been approved by our apps marketplace team.
I’m afraid this installation URL doesn’t yet support customization. However, I do think it would be a good idea to submit your app for our marketplace anyways. Not only will this be good for publicity and exposure, but will also alleviate this customization difficulty with the share installation URL!