Copy from one cell and paste to another sheet based on length of another column

Hi I am using the code below to copy and paste from one column to another based on cells containing data in column A, what I need to do it copy data from a single cell (J2) in sheet1 and paste into next empty cell in column A on sheet 2 based length column B (cells containing data)

Sub FillDown()
Dim LastRow As Long

With ActiveSheet

LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
.Range("D2").AutoFill .Range("D2").Resize(LastRow)
End With
End Sub