@app.route("/login", methods=["GET", "POST"])
def login():

    if request.method == "POST":

        username = request.form.get("username")
        password = request.form.get("password")

        if (
            username == USERNAME and
            check_password_hash(PASSWORD_HASH, password)
        ):
            session["logged_in"] = True
            return redirect("/")

  
    <form method="post">

        Username:<br>
        <input name="username"><br><br>

        Password:<br>
        <input type="password" name="password"><br><br>

        <button type="submit">Login</button>

    </form>