Can someone help me please

Hi all im trying to get a script that depending on the value of field name (code)  it

would ask me the percentage amount and then add that percentage amount to the total of another field 

basically what i'm trying to get is

if code = abc then ask to input amount of percentage then add amount of percentage to total

so if percentage =10 and total = 1000 then the amount of total would be 1100

any help would be greatly appreciated

Thanks