how to get it [an API token]? Is it the same which I can generate here → Atlassian account ?
But I don’t follow between prior discussion and your recent assertion:
According to the documentation to make a request to the REST API my app has to send the API key.
For Connect, the Security for Connect apps docs explain:
When communicating server-to-server with the Atlassian host product your app uses a JWT token to access protected resources including most of the REST APIs. When you use the frameworks, this is handled for you.
If you are not using the frameworks, you can use JWT libraries to construct a token. See Creating a JWT token and the Atlassian-supported claims that you need to construct.
You should not need or use API tokens for a Connect App.