how to insert data in a custom field using api?

I want to record the start and end date of an event for third parties and would like to know how to save this data using api

Captura de ecrã 2020-12-03, às 18.18.34

I’m a little surprised you got no answers yet.

Anyways I found the API to be somewhat lacking and buggy in v12, so I would recommend to work directly against the database for this kind of need.

It would be nice to improve the API further and have better testing of it. If anyone is interested I made some issues a while back in github.