I'm trying to apply a formula to a range that a user can select.
Basically I have a user form where the user can select a range of data
to be processed as well as an output cell where there the
converted/processed range is put.

Once the input ranges and output cell has been chosen I need to apply a
formula to every cell in the input range and put it in the output cell.

I have been successful in doing this where the input range is only a
single selection but have been unable to get it to work with more
multiple selections as the input range. Here is my code:

Dim r As Range
Dim r2 As Range

' Input and output ranges in userform
Set r = Range(refInput.Value)
Set r2 = Range(refOutput.Value)

r.Copy
r2.PasteSpecial Paste:=xlPasteValues

Selection.SpecialCells(xlConstants).Formula = "--- formula goes
here ---"

' Get rid of formulas so user does not see them.
Selection.Copy
Selection.PasteSpecial Paste:=xlPasteValues

Any way to get my code to work with multiple selections.