How to add a button programmatically in VBA next to some sheet cell data?

I think this is enough to get you on a nice path:

Sub a()
  Dim btn As Button
  Application.ScreenUpdating = False
  ActiveSheet.Buttons.Delete
  Dim t As Range
  For i = 2 To 6 Step 2
    Set t = ActiveSheet.Range(Cells(i, 3), Cells(i, 3))
    Set btn = ActiveSheet.Buttons.Add(t.Left, t.Top, t.Width, t.Height)
    With btn
      .OnAction = "btnS"
      .Caption = "Btn " & i
      .Name = "Btn" & i
    End With
  Next i
  Application.ScreenUpdating = True
End Sub

Sub btnS()
 MsgBox Application.Caller
End Sub

It creates the buttons and binds them to butnS(). In the btnS() sub, you should show your dialog, etc.

Suppose your function enters data in columns A and B and you want to a custom Userform to appear if the user selects a cell in column C. One way to do this is to use the SelectionChange event:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim clickRng As Range
    Dim lastRow As Long

    lastRow = Range("A1").End(xlDown).Row
    Set clickRng = Range("C1:C" & lastRow) 'Dynamically set cells that can be clicked based on data in column A

    If Not Intersect(Target, clickRng) Is Nothing Then
        MyUserForm.Show 'Launch custom userform
    End If

End Sub

Note that the userform will appear when a user selects any cell in Column C and you might want to populate each cell in Column C with something like "select cell to launch form" to make it obvious that the user needs to perform an action (having a button naturally suggests that it should be clicked)