Throw a Popup with input enabled fields?

Hello,

I have a requirement that the moment user hits the SAVE button on ME51N/ME52N screen (Purchase requisition - Doc type is my_PR) process, I need to throw a popup, wherein 2 input enabled fields will be there for user input.

Once user enters the data on popup & click OK then, I need to save it to a custom table

Pl. let us know how can achieve this? Where I need to put my code for to call a custom popup (I guess, I need to create a new screen for popup in SE51 and call it on SAVE hitting) thank you