Attach a custom field to a job using the API

[andreywis]

Can the API be used to attach a custom field to a job?

What is the process for attaching a custom field that is not shown for every job to a specific job?

In the API documentation, I can observe the following note:

Note: The required_customfields property can't be modified through the API, but can be modified through the TSheets web app when logged in as an admin user.

Is it safe to assume that this is currently not possible? Attaching a custom field to a new job created through the API is only possible by checking 'Show for all jobs'?