How to pass used_id to Watson-AssistantV2 node?

Hi! I have been trying to send the user_id to the Watson Assistant V2 node in different ways but I could not find anyone that worked.

I tried:

msg.user_id
msg.params.user_id
msg.params.context.user_id
msg.params.context.metadata,user_id
msg.params.context.glogal.system.user_id
msg.addiltinal_context.glogal.system.user_id

In the node documentation there is no mention of the user_id, only session_id.
user_id is important as the billing of the assistant is measured by the monthly active users.
I cannot add it in the payload as payload only allow a "string" as the input text from the client.

Many thanks!!