How to make the Bot start the conversation (in Slack) for new user

the bot is installed and working perfectly in Slack… but i always start the converstaion by saying (“HI”, “Hello” …etc)

User: Hi

Bot: Hello {Username}, How are you ?

is there a way to make the bot start the conversation(whene a new user install the app), by activation the intent name (/start_onboarding) or something similar.

any suggestions or ideas are very welcome, thank you

Python: 3.7

Rasa: latest version

Botfront: latest version