Search in Custom Fields only

I created a custom field (Format: Number) where I store all the tickets number of my service providers (eg. 125 or 101134).

I would like the search function to be able to select a custom field to limit the search for this field or fields, otherwise the search function will never find such a generic number in the proper task.

I  also checked the Search Commands but there is nothing which allows me to do what I need.