Set default category via query string

gistfile1.txt

add_filter( 'submit_job_form_fields', 'custom_submit_job_form_fields' );

function custom_submit_job_form_fields( $fields ) {
	if ( isset( $_REQUEST['job_category'] ) ) {
		$fields['job']['job_category']['default'] = sanitize_text_field( $_REQUEST['job_category'] );
	}
	return $fields;
}