Hi Yoann,
This can vary based on which platform you are using for your bot but as an example the Facebook Messenger Bot is able to use a m.me link with a ref keyword.
The support docs on Creating a Facebook Messenger Bot has more information on using this feature but the general idea is that you use the m.me link with a unique ref to keep track of where your leads have accessed the bot from.
An example of referral links would look something like:
m.me/8733435?ref=GoogleAds
m.me/8733435?ref=Facebook
m.me/8733435?ref=Website
etc.
The ref links are picked up as Metadata in your Flows which provides you with the opportunity to capture that information and even filter certain flows to only run for set ref links.
Hope that helps. Let me know if anything is unclear.
Thanks,
Dan