Formula In Macro To Join Various Cell Values

apSheet.Range("A2").Formula = "=D2&E2&F2"

I am trying to do the same kind of formula with a user form where the user is picking the range for the needed columns. I am not sure how to make it work.

Private Sub test()

'declare variables
Dim wb As Workbook
Dim iSheet As Worksheet
Dim apSheet As Worksheet
Dim glSheet As Worksheet
Dim x As Long
Dim apA, apB, apC, apD, apE, apF, apG, apH, apI, apJ
Dim LstAPRow

'set variables
Set wb = ThisWorkbook
Set iSheet = wb.Worksheets("Instructions")
Set apSheet = wb.Worksheets("AP Query")

With apSheet
LstAPRow = Range("A65536").End(xlUp).Row
End With..............................................