This is one attempt. I'm wondering, can I include a public sub or public function INSIDE a private function?

Private Sub RequeryMainForm()

    Dim SQL As String, WhereStr As String
          
    MyFilterCode
    MsgBox MyFilterCode
    SQL = "SELECT * FROM BudgetQ"
    
    If WhereStr <> "" Then
        SQL = SQL & " WHERE " & WhereStr
    End If
    
        SQL = SQL & " ORDER BY TransactionDate"
    
    
    Me.Register.Form.RecordSource = SQL
    SQL2 = SQL

You will need to send several parameters to the procedure. Since I don't know your database, I used the one Richard built in the video and modified it. The screenshot below should give you a good idea how to proceed.

Richard's code is designed to be run for that one form only, and if you want to put that code in a public procedure, that code needs to be re-written to run for other forms.  

For instance, at 9:30 of the extended cut video, in this part of the code:

     If FirstNameSearch <> "" Then ...

"FirstNameSearch" is equivalent to Me!FirstNameSearch, which only refers to that particular control on that form.  Outside of the form, that reference has no meaning (and has no value), and that's why your public procedure didn't work.  Your probably didn't use "Option Explicit", so Access thinks "FirstNameSearch" was a public variable, one that had no value.  So that's why your public function returned blank.

To make this work in a public function for ANY form that has a "FirstNameSearch" control, your public function needs to accept a reference of the form (F, in the example below) that needs to use this function:

     Public Function MyFilterCode(F As Variant) As String
          Dim WhereStr As String
          If F!FirstNameSearch <> "" Then
               WhereStr = ...
               ...
          End If
          MyFilterCode = WhereStr
     End Function

Your form, or any form, will then be able to call this function in this manner:

     WhereStr = MyFilterCode(Me)

"Me" is a reference to your current form.  It is passed to MyFilterCode, so it knows the code is being run on that form.

Thank you Kevin!  I think I now know what I need to do.

I want to say thanks again to both Kevins. Unfortunately I can't get either to work, so for now I guess I will just stay with duplicate code. I've not given up on your suggestions though, and will continue to try to get one of them to work.