Now go to the Visual Basic in Developer Tab.
Double click on This workbook and paste the below vbu code.
Private Sub Workbook_Open()
'Sub FillEmptyBlankCellWithValue()
Dim cell As Range
Dim InputValue As String
On Error Resume Next
InputValue = InputBox("Enter value that will fill empty cells in selection", _
"Fill Empty Cells")
For Each cell In Selection
If IsEmpty(cell) Then
cell.Value = InputValue
End If
Next
End Sub
Now click this

Remember to save your file as Micro enable format in .xlsm format.
All set!!
Sample Add Zero in Blank Cell