Press ALT
Alt+F11
F11 to open the VBA screen.
This code should copy down the formula, just change the Range to the correct cells
Range("B1:B10000").FillDown
Range("B1:B10000").FillDown
Edit
Or. Or just add the formula with VBA
ActiveSheet.Range("B1:B10000").formula = "=A2+2"
ActiveSheet.Range("B1:B10000").formula = "=A2+2"