Set up sending events from Zabbix according to the instructions
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/media/redmine?at=release/5.4
Add custom field, use it in Zabbix action:

customfield_2 => {EVENT.ID}

events are not sent, but in the server logs I see the following

[ Redmine Webhook ] ERROR: Request failed with status code 403 Check debug log for more information.

If the redmine account on behalf of which zabbix works, add administrator rights - there is no problem

Please add this note to the documentation