OneSignal have a feature for in-app chat messages?

Integrations

I have a chat feature on my App and I want every message to play in-app sound (e.g. Messenger sound when there's new message)

Do they have a feature for that or not?

(NOTE: I'LL USE IT FOR CHAT FEATURE OF MY APP)

What have you tried so far?

I completed the FCM guide, I'm getting the ff_user_push_notifications and fcm_tokens. I'm having a hard time implementing it on in-app Chat Messaging Feature to receive a custom sound when there's a new message.

Did you check FlutterFlow's Documentation for this topic?

Yes