Search on hidden meta custom fields

Hi,
I use the WP Job Manager plugin on the site for customers to submit jobs. With this plugin, custom fields are hidden ones. The explanation from the WP Job Manager people is below
“Note, the field name is prepended with a ‘_’. This is because Job Manager makes your new fields hidden meta by prepending theme with an underscore”

The issue is that, using the WP Extended Search plugin, I cant see or search on these hidden meta custom fields.

Any help is much appreciated.