single line text insert address variable

I created a custom taxonomy called Location (nonhierarchical). The Gravity Forms + Custom Post type plugin lets set a single line text field where any user inputs would add to that taxonomy.

What I want to do is have the text field hidden and pass the City value from an address field into the single line text field.

I tried setting the default value to the {Address (City):1.3} variable but the field came back empty.

Can this be done another way?

I added this to my functions.php and it's still coming back blank:

add_action("gform_pre_submission_3", "pre_submission_handler");
function pre_submission_handler($form){
    $_POST["input_18"] = $_POST["input_1.3"];
}

where field 18 is the hidden text field and 1.3 is the city part of the address field.

Edit:
I went over some other related posts. I should have had "input_1_3". It's working now. Thank you for the help.