How to test form submission

I have a login form with some html that users fill in:

<form method="post" enctype="multipart/form-data">
        <div class="form"><input name="username"> </div>
        <div class="form">
        <button class="btn btn-primary btn-block" type="submit">Login</button>

I would like to test that the username is being received correctly. I am able to send a multipart form however this is contained within the body of the request not as a postform.

    form := url.Values{}
    form.Set("username", "Bob")

    // Create request
    req, _ := http.NewRequest("POST", "/login", strings.NewReader(form.Encode())) // sends as multipart form in request body
    req.Header.Set("Content-Type", "application/www-form-urlencoded")
    testRouter.ServeHTTP(rr, req)

How do I make a request with the form data attached as a Postform so that I can extract it c.Postform("username") from the backend as if submitted via the browser?