Signal Webhooks send document fields


I have set up a custom signal that is triggering within ELK as i want.
I would like to use the Webhook function to trigger some ex ternal actions.
In order to do this though i need to send some of the document fields in the body of the webhook.

I have tried to search on how to do this but can not find any documents/guidance.
Is this even possible ?

If it is can somone provide me with what should go in the moustash parameter to send @timestamp for example?

Thanks in advance