Source

File: wp-admin/includes/plugin-install.php

function install_search_form( $deprecated = true ) {
	$type = isset( $_REQUEST['type'] ) ? wp_unslash( $_REQUEST['type'] ) : 'term';
	$term = isset( $_REQUEST['s'] ) ? wp_unslash( $_REQUEST['s'] ) : '';
	?>
	<form class="search-form search-plugins" method="get">
		<input type="hidden" name="tab" value="search" />
		<label class="screen-reader-text" for="typeselector"><?php _e( 'Search plugins by:' ); ?></label>
		<select name="type" id="typeselector">
			<option value="term"<?php selected( 'term', $type ); ?>><?php _e( 'Keyword' ); ?></option>
			<option value="author"<?php selected( 'author', $type ); ?>><?php _e( 'Author' ); ?></option>
			<option value="tag"<?php selected( 'tag', $type ); ?>><?php _ex( 'Tag', 'Plugin Installer' ); ?></option>
		</select>
		<label class="screen-reader-text" for="search-plugins"><?php _e( 'Search Plugins' ); ?></label>
		<input type="search" name="s" id="search-plugins" value="<?php echo esc_attr( $term ); ?>" class="wp-filter-search" placeholder="<?php esc_attr_e( 'Search plugins...' ); ?>" />
		<?php submit_button( __( 'Search Plugins' ), 'hide-if-js', false, false, array( 'id' => 'search-submit' ) ); ?>
	</form>
	<?php
}