dynamically create hiddenfield based on drodown list value

Question

create hiddenfield based on dropdown list selected value 

i have a grdview with add new row button in each row i have dropdown control like below image  my req: when i select dropdown based on drodwn selected value create  one hidden field as store some value  if already selected value in another row of gridview dont create hidden field use already created hiddenfield whicih is previously selected