EventSub will send a noticitaion like this:
See Channel Follow Webhook Notification Example on the link I linked:
{
"subscription": {
"id": "f1c2a387-161a-49f9-a165-0f21d7a4e1c4",
"type": "channel.follow",
"version": "1",
"status": "enabled",
"cost": 0,
"condition": {
"broadcaster_user_id": "1337"
},
"transport": {
"method": "webhook",
"callback": "https://example.com/webhooks/callback"
},
"created_at": "2019-11-16T10:11:12.123Z"
},
"event": {
"user_id": "1234",
"user_login": "cool_user",
"user_name": "Cool_User",
"broadcaster_user_id": "1337",
"broadcaster_user_login": "cooler_user",
"broadcaster_user_name": "Cooler_User",
"followed_at": "2020-07-15T18:16:11.17106713Z"
}
}
The user that followed is this section of that JSON blob.
"user_id": "1234",
"user_login": "cool_user",
"user_name": "Cool_User",