how to add extra input fields at post and display in the thread title

[radiofranky]

HI,
I was wondering how to add additional input fields in the "post new thread" page?
and pass those input fields variables into thread title.

for example,

input a = "$10"
input b= "$20"

in thread title ="1 gallon of ice cream - 50%off pay only $10 instead of $20"

thanks