grav-theme-libretic/templates/partials/simplesearch_searchbox.html.twig

<input type="text" placeholder="Search..." value="{{ query }}" data-searchsidebar-input="{{ base_url_relative }}{{ config.plugins.simplesearch.route}}/query" />
<script>
jQuery(document).ready(function($){
    var input = $('[data-searchsidebar-input]');

    input.on('keypress', function(event) {
        if (event.which == 13 && input.val().length > 3) {
            event.preventDefault();
            window.location.href = input.data('searchsidebar-input') + '{{ config.system.param_sep }}' + input.val();
        }
    });
});
</script>