How can I program in VBA to require that a user inputs a note (I want a blank
note to pop up) when they change a number in a cell?

For example, if I have cell B10 = 1,500 or =trend(B1:B9,A1:A9,B10:B20) and
someone decides to type in a different number into cell B10, I want Excel to
first open a pop up window asking 'What is the reason for the change?",
second not allow the person to continue until they input a reason in the pop
up box and finally save the note in the cell for future users to read.

Thank you.

Adam