functions.php

<?php

// @link https://searchwp.com/documentation/knowledge-base/engine-select-dropdown/

// ***** UNCOMMENT THIS IF YOU WANT TO USE SHORTCODES IN WIDGETS
// add_filter( 'widget_text', 'do_shortcode' );

// Output a SearchWP Engines dropdown
function my_searchwp_shortcodes_inject_engines() { ?>
	<p class="searchwp-shortcodes-engines">
		<?php
			$engines = \SearchWP\Settings::get_engines();
			$current_engine = isset( $_GET['swpengine'] ) ? esc_attr( $_GET['swpengine'] ) : 'default';
		?>
		<select name="swpengine" id="swpengine">
			<?php foreach ( $engines as $engine_name => $engine ) : ?>
				<option value="<?php echo esc_attr( $engine_name ); ?>"
					<?php selected( $current_engine, $engine_name ); ?>>
					<?php echo esc_html( $engine->get_label() ); ?>
				</option>
			<?php endforeach; ?>
		</select>
	</p><?php
}

add_action( 'searchwp_shortcodes_after_input', 'my_searchwp_shortcodes_inject_engines' );