Pop up box after selection of specific items from a drop down list.

Hello,

I have a populated drop down box and I would like the option of when a item is selected in the drop down, a pop up box will be displayed that gives a option to select item A or item B. Once selected it will fill in another box on the forum. Any help on the matter would be greatly appreciated. Thank you in advance.

Example: Dropdown selection= Item A

Pop up: 22.5 or 26.5

Field A: based on selection show 22.5 or 26.5