An ideal solution is to filter the locations that appear in IT Shop using Web Designer.
In Web Designer choose Edit | Configure Project | Column-dependent references...
Click the green plus symbol to create a new entry.
In the example of Locations select the Person table, column Person.UID_Locality. Locate the correct component you want to make changes to in your form and select it in the "Component" drop down. For example, VI_Edit_FK.
Then in the "Foreign key filter" use the wizard to build a filter for the Locations you want to appear. For example, to filter those that begin with the letter 'D' you could enter: "(isnull(Ident_locality, N'') <> N'%d'). Using the WHERE clause wizard is the easiest way to do this.
Test your changes in the Web designer preview to confirm you get the desired results.