message box when cell is blank

Column A (any row):

might have text or might not

Column B (any row):

text is added manually

If you try to enter text into column B (in any row) AND the adjoining cell in column A (in any row) is blank then pop up a warning.

So column A, row 4 has text. You can enter text into Column B, row 4
If column A, row 4 is blank and you try to enter text into column B, row 4 it throws up a warning.