In case where a code and a name is maintained for an entity, (for example for Employee there may be an Employee Code and Employee Name) and we want to show both the ID and name in a link field, we can make a formatter.

Example:

frappe.form.link_formatters['Employee'] = function(value, doc) {
    if(doc.employee_name && doc.employee_name !== value) {
        return value + ': ' + doc.employee_name;
    } else {
        return value;
    }
}

Notes:

1. Both the primary key (name) and the descriptive name (e.g.employee_name`) must be present in the document. The descriptive name field could be hidden
2. This needs to be loaded before the document is loaded and can be re-used for all forms. You can also add it in build.json