Is it possible to register some Azure AD app (which is used just for OAuth for other desktop apps) by hand in Azure env on "Enterprise applications" page?

Question

We develop applications which connect via EWS (and Graph) API to Office 365 data like Mails / Calendars / Contacts folders.
After OAuth 2.0 was introduced as authenticate method for EWS we had to register OUR App in Azure AD once.
So now our customers should pass the one-time procedure to accept our app in their Azure env via browser window.
Question is - if it's possible to register our app in their Azure env by hand on "Enterprise applications" page?
I see menu "New application" but it lists apps from Azure AD Gallery only.
So is it obligatory to publish our app to Azure AD Gallery in order customer can add it? Or it's possible to use some other method (for example we could send to customer JSON-manifest of our app) ?
If the registration in Azure AD Gallery is obligatory - how to do it correctly?
I read this link
https://learn.microsoft.com/en-us/azure/active-directory/develop/v2-howto-app-gallery-listing
But is our app satisfied the described requirements?
Because in fact it does nothing - just is used for OAuth for other apps.
Can someone to explain it more clearly?