You should check the InputBox function
Basically, it displays a prompt in a dialog box, waits for the user to input text or click a button, and then returns a string containing the contents of the text box.
So, for your code, it would be like this :
Sub Delete_Rows()
Dim selectedValue As Integer
selectedValue = InputBox ("Please, enter a number", "Input for deleting row", Type:=1)
'Prompt 'Title 'Value type (number here)
Dim rng As Range, cell As Range, del As Range
Set rng = Intersect(Range("A2:J707"), ActiveSheet.UsedRange)
For Each cell In rng
If (cell.Value) = selectedValue _
Then
If del Is Nothing Then
Set del = cell
Else: Set del = Union(del, cell)
End If
End If
Next cell
On Error Resume Next
del.EntireRow.Delete
End Sub