How to mock a local variables using mockito/powermock

Question

I have a function as follows :

private Response getHttpResponse(String url) {
    WebTarget target = client.target(url);
    Invocation.Builder requestBuilder = target.request().accept(StandardCharSets.UTF_8.toString());
    if (headerMap != null) {
        for (final Map.Entry<String, Object> header: headerMap.entrySet()) {
            requestBuilder.header(oneHeader.getKey(), String.valueOf(header.getValue());
        }
    }
    Invocation request = requestBuilder.buildGet();
    Response response = request.invoke();
    return response;
}

I've spent way too much time on this. So I thought I should share my findings. Here my question is how to mock such local variable(request), so that I can mock invoke method response.