How to do it...

1. Open your AL project folder in Visual Studio Code.
2. In Visual Studio Code's Explorer pane, right-click and create a new file named Customers With Television Shows Filter Token.al. In the Editor tab, create a new codeunit object, and then add the following code:

codeunit 50103 "Customers With Shows Filter"
{
    [EventSubscriber(ObjectType::Codeunit, 
    Codeunit::TextManagement, 'OnAfterMakeTextFilter', 
    '', true, true)]
    local procedure OnAfterMakeTextFilter(var Position: Integer; 
    var TextFilterText: Text)
    var
        Customer: Record Customer;
        CustomerTelevisionShows: Record "Customer Television Show";
        CustWithShowsTokenTxt: Label 'CUSTWITHSHOWS';
        MaxCount: Integer;
    begin
        if StrPos(UpperCase(CustWithShowsTokenTxt), 
        UpperCase(TextFilterText)) = 0 then
            exit;

        MaxCount := 2000;
        TextFilterText := '';

        if Customer.FindSet() then begin
            repeat
                CustomerTelevisionShows.SetRange("Customer No.", 
                Customer."No.");
                if not CustomerTelevisionShows.IsEmpty() then begin
                    MaxCount -= 1;

                    if TextFilterText <> '' then
                        TextFilterText += '|';

                    TextFilterText += Customer."No.";
                end;
            until (Customer.Next() = 0) or (MaxCount <= 0);
        end;
    end;
}

1. Now, let's test it! Press F5 to build and publish your application. Navigate to the Customers list page and click the  icon to show the filter pane. Enter a filter value of %CustWithShows into the No. field and press Tab. Note the value that you enter is not case-sensitive:

The list should get filtered down to only show the customers that have television shows defined. If your list is empty, then release the filter, enter some television shows for a customer or two, and try the filter again.