I've been asked several times to add custom lookups to user defined fields in GP.
Clients want to utilize user defined or custom fields, but they want to have the values validated, and they want to allow the user to select a valid value from a list. Makes sense.
I briefly checked out the drop down and list box controls available in Modifier, but they are pretty limited, and aren't really oriented towards displaying dynamic result sets from a database query.
So I ventured out and created my own custom lookup window in VBA.
As a side note, my biggest gripe with the standard GP lookup windows is their lack of a simple, "real-time" search field. By "real-time", I mean that I want the lookup window to instantly filter the displayed records based on what I type in the lookup search box--as I type it. And I want to have the window simultaneously search multiple fields at the same time--not just one field. For standard GP lookup fields, Rockton SmartFill does an excellent job in this regard, but sadly, in my experience, few clients have been willing to pay for SmartFill.
I'm going to show an example of a lookup that displays Sales Prospect records. In this case, the client called prospects "End Users", so it is called the "End User Lookup" window.
I first created a custom User Form in VBA, and added a Search field at the top. Because VBA is pretty limited in terms of controls, I had to use a ListBox to display my results. Finally, I have a Select and Cancel button.

When the form loads, I utilize UserInfoGet to have it query the GP company database and retrieve all prospects. I store those prospects in a client-side disconnected ADO recordset so that they remain in memory. I then have a simple procedure that displays the recordset results.

When a user types in the Search box, I send the equivalent of a SQL WHERE clause to the ADO recordset Filter property. This dynamically changes the records available in the recordset, allowing me to quickly redisplay only matching records as the user types each character.

As you can see in this example, when I type "wav" in the search box, I am simultaneously searching on the ID, name, and city of the prospect records,…