Follow this guide: https://dev.twitch.tv/docs/authentication/getting-tokens-oauth#oauth-authorization-code-flow
That’ll take you through the auth process to create a User Access Token, and a Refresh Token. This step can’t be automated, you have to go through this process once so that you can grant your app the specific permissions you need, which in this case will be the channel:read:subscriptions scope to grant you access the list of subscribers to your channel.
Once you’ve got the tokens, you can automatically refresh them as they expire so from this point on you can automate it all, it’s just that initial auth step that requires the user to actually go through Twitch to grant you the permission to access the data.