Is it possible to make a macro to the "reedem CD Key prompt"?

steam://open/activateproduct

it autohotkey's to the position to do the stuff then pastes the keys in i believe, its automated and does it for you just have to store the keys in the text file for the program.

its only of use really if you have an awful lot of keys to activate at once

from the link above:

steam_open_activation_window(){             ;steam open the activation window
    Run steam://open/activateproduct
    applog("activated steam url to open activation window")
    Sleep,100
    return
}

so according to that it would be:
steam://open/activateproduct

;)