From the documentation we have just:
To export information into a ServiceNow custom field, add a parameter with the custom field ID as a key (if you need help finding custom field ID, see this page in ServiceNow community).
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/media/servicenow?at=release/6.0
The link provided there is not clear.
Judging by the text of the script, it is expected that all custom parameters are named as columns in system tables (starting with u_)

lse if (key.startsWith('u_')) { fields[key.substring(2)] = params[key]; }

all other parameters are ignored
But there are some custom parameters that should be passed without u_. For example:
"assignment_group", "impact", "channel"
Please add the ability to use such custom parameters and clarify in the documentation how to use them.

Regards, Elina