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)