Question on Saving the credit card number

I have a need for the credit card number (not masked) and need to send it on to another system. Is there a way to accomplish this? I am wondering which hook to use and if anyone might have a snippet of code. Specifically, I would like to get the unmasked credit card number and save it to a hidden field for now.

I tried this one to no avail:

add_filter("gform_pre_submission_filter", "add_cc");
function add_cc($form){

    //get the credit card
    $cc  = $_POST["input_91.1"] . ",";
    $_POST["input_98"]=$cc;
    //returning modified form object
    return $form;
}