Add Custom Field To All Jobs

To add new fields, use functions and/or filters inside a functionality plugin:

https://wpjobmanager.com/document/editing-job-submission-fields/

https://css-tricks.com/wordpress-functionality-plugins/

You can also use a third-party plugin such as WP Job Manager Field Editor:

https://plugins.smyl.es/wp-job-manager-field-editor/

… which has a nice GUI for editing fields.

Thanks!