Add webhooks from an external application

[djachim]

I want to be able to ask the owner of a Shopifystore to enter the client_id/secret on my application.
Using that information, I want to add webhooks for assorted events to post data.

I found https://shopify.dev/docs/apps/build/authentication-authorization/session-tokens/set-up-session-token... but this requires a session_id from a Shopify application.
My application is not part of Shopify.  How can I use client_id/secret from outside of Shopify to access the APIs?