Change the place holder for the default search widget.

gistfile1.txt

function custom_search_form( $form ) { 
    $form = '<form role="search" method="get" class="search-form" action="' . esc_url( home_url( '/' ) ) . '">
    <label>
        <span class="screen-reader-text">' . _x( 'Search for:', 'label', 'astra' ) . '</span>
        <input type="search" class="searce-field" ' . apply_filters( 'astra_search_field_toggle_data_attrs', '' ) . ' placeholder="' . apply_filters( 'astra_search_field_placeholder', esc_attr_x( 'Your place holder', 'placeholder', 'astra' ) ) . '" value="' . get_search_query() . '" name="s" role="search" tabindex="-1"/>
    </label>
    <input type="submit" class="search-submit" value="Search">
</form>';

 return $form;
}

add_filter( 'get_search_form', 'custom_search_form' );