Adding first-name and last-name from contact for to a single Mailchimp Field

Hi, another challenge ??
Can I add First Name and Last Name separate fields to a single field in Mailchimp?

With your code do you think this is right? The field “your-fname” is mandatory but “your-lastname” is not. So it might be empty.

function additional_contact_form_7_data( $merge_variables, $cf7_variables ) {
    if ( isset( $cf7_variables['your-fname'] ) ) {
        $first-name = filter_var( $cf7_variables['your-fname'], FILTER_SANITIZE_STRING );
        $last-lastname = filter_var( $cf7_variables['your-lastname'], FILTER_SANITIZE_STRING );
        $fullname = $first-name . " " .  $last-name;
        $merge_variables['FNAME'] = $fullname;
    }
}