// WP Fusion Exctra class Filter
function custom_wpf_query_args( $query_args, $method ) {

    if( 'get_contact_id' == $method ) {

        $query_args['q'] = str_replace( 'Email', 'Email__c', $query_args['q'] );

    }

    return $query_args;
}

add_filter( 'wpf_salesforce_query_args', 'custom_wpf_query_args', 10, 2 );