mitmproxy/libpathod/templates/request_previewform.html

<form class="form-inline" method="GET" action="/request_preview">
    <input 
        style="width: 20em"
        id="spec"
        name="spec"
        class="input-medium search-query"
        value="{{spec}}"
        placeholder="method:path:[features]"
    >
    <input type="submit" class="btn" value="preview">
</form>