When a value is writing (X2) I need a pop up text box to write the value to the next cell

I am going to explain with the example, I think it is easier. What I need is a macro to run and analyze columns F, I, L, O, and so on and when I write X2, a pop up text box appears. On that pop up text box I should be asked "How many hours I want to enter" and that value is added to the cell on the right. For instance, when I write X2 in cell I9, and write 8 on the pop up, the number 8 should be entered at J9...

example