Add content Parameters user_id to Kaltura Http Notification Template

Hi there, I would like to add user_id content parameter to an existing KalturaHttpNotificationTemplate
When I run eventNotificationTemplate.update, it shows success but the new content parameter was not added.
Thanks for your help!

A little more info - I would like to use the user_id of the user that is subscribed to the category
as a parameter in the notification. I’m using

<type>httpNotification.Http</type>
<eventType>5</eventType>
<eventObjectType>37</eventObjectType>

So I get
“entry_id”,
“entry_name”,
“category_id”,
“category_name”

as content parameters automatically.
I would also like to be able to use the user_id of the user that is subscribed to the category
as a parameter. I’d greatly appreciate ideas on how to do this!
Thanks so much
Anna

I found the answer - this is not currently possible.