Filters

realia_filter_fields

add_filter( 'realia_filter_fields', 'custom_filter_field' );

function custom_filter_field( array $fields ) {
    $fields[] = array(
        'custom-search-field' => 'Custom Search Field',
    );

    return $fields;
}

realia_filter_field_names

add_filter( 'realia_filter_field_names, 'custom_filter_field_name' );

function custom_filter_field_names( array $names ) {
    $names[] = 'custom-search-field';

    return $names;
}

realia_payment_gateways

add_filter( 'realia_payment_gateways', 'custom_gateway' );

function custom_gateways( array $gateways ) {
    $gateways[] = array(
        'id'        => 'custom-gateway',
        'title'     => 'Custom Gateway',
        'content'   => 'CUSTOM-FORM-HERE-IF-NEEDED',
        'proceed'   => true,
    );
}