Customize the relationship field list query

I need to make the Relationship Field Search functionality to search also in 3 other Custom Fields and then display the result if it match any of them.
In other words: I have Title where is the name of the person and 3 custom fields with phone, email, etc.
It would be great if when I type in the search box the list with related posts is filtered by any of these fields.

I found the flowing in the documentation, but somehow can not came up with the idea how to use the search Variable for value.

http://www.advancedcustomfields.com/resources/tutorials/customize-the-relationship-field-list-query/

Any suggestions will be highly appreciated.

Thanks

https://wordpress.org/plugins/advanced-custom-fields/