How to send an HTTP request when recording a Redmine task with the Closed status?

Question

Ruby on Rails

Hello!
The task is as follows: when writing a task in Redmine,
a) check if the additional field
is filled b) if it is filled, send an HTTP request to another service
If I understand correctly, I need to write a plugin for the event "controller_custom_fields_edit_after_save" or it is not clear), there to check additional. field (how to access it?) and send a request.
Please tell me, if anyone knows how to attach to the right hook, check the add. field and where to write it.