Append to last name using gform_pre_submission

I'm trying to append a description from a text field to a name field using gform_pre_submission using this code below:

add_action("gform_pre_submission_38", "desc_append");
function desc_append($form){
    $_POST["input_3.6"] = $_POST["input_3.6"] . ' ('.$_POST["input_2"].')';
}

My goal is to end up with First Last (Description) in the name field.

As I'm understanding, this should append the description to the last name, but it's not working. When I use the same code to append a text field to another text field (input_1) it is working fine, so I know I'm close. I assume it is something to do with the input_3.6 for the Last Name. Any help is appreciated.