Add formula to cell that refers to first empty cell in another row.

Hi, I got this peace of code:

Worksheets("Invoer").Range("A" & Rows.Count).End(xlUp).Offset(1, 3).Value = "=A5"

I want to alter it in such a way that the '5' at the end is actually the row number of the first cell in column A that does not contain any value.

Any help there?