How to change optional fields to compulsory?

Hi @sheyvr here is an example of adding a field: https://wpjobmanager.com/document/tutorial-adding-a-salary-field-for-jobs/

For changing the requirements for fields, you can do this either with using the submit_job_form_fields filter or altering the code: https://github.com/Automattic/WP-Job-Manager/blob/c35a30616947996570927259426f35f97f4b7944/includes/forms/class-wp-job-manager-form-submit-job.php#L160