how to add a Formula in VBA

I have a spreadsheet that I want to add a formula to when i run some VBA.   Basically, i want this formula to be entered when i run the code:

=IFERROR(INDEX(SHEET1!A:C,MATCH(Q2,SHEET1!A:A,0)+COUNTIF(SHEET1!A:A,Q2)-1,2),"")

I know how to write it manually, i just don't know how to do it via VBA.

I tried this:

Sheets("This Sheet").Select
Dim lastrow as Long
lastrow = [Q8000].End(xlUp).Row
For i = lastrow To 1 Step -1
If i = 1 then
GoTo Done
End If

If Cells(i, "Q").Value <> "" Then
Cells(i, "B").Value = "=IFERROR(INDEX(SHEET1!A:C,MATCH(Q2,SHEET1!A:A,0)+COUNTIF(SHEET1!A:A,Q2)-1,2),"")"
End If

Next i
Done :

As you probably know, that didn't work.    I have the same problem with vlookup like this:

=vlookup(I2,sheet1!A:AP,5,False)

That didn't work either.  I am not good at the formula thing in VBA.

Thanks in advance