How can we invoke the macro lifecycle event in a static app? In other words - How to implement a server (express server preferred) in a static app that uses GCP?

We are facing a problem in migrating the Image slider macro using migration assistant. This is a static app and it doesn’t have any backend/server.
It uses Google /Cloud Platform(GCP) and ngrok to run/deploy the app. GCP is dependent on python 2.
For migration, we have implemented a listener on the server side. In the cloud, we are trying to register a webhook when the macro is installed using the “installed” lifecycle method. For that, in atlassian-connect.json we added
“lifecycle”: {
“enabled”: “/enabled”,
“installed”: “/installed”
We tried to implement the backend rest server using python but ran into a problem -

  1. POST: /enabled - invoke migration API PUT method to register the webhook (/rest/atlassian-connect/1/migration/webhook)
    this API requires an authorization token. We are facing a problem in getting this token. From where and how to get this token?
    We are stuck at this point and will appreciate your help.