用自定义URL替换表单操作

通过学习HTML和JavaScript,我试图根据通过表单提交的输入,为API请求形成一个自定义URL。我已经能够创建大多数URL,但我在让URL字符串连接到“?”之前遇到了一个问题提交操作时。我这么做仅仅是基于文档和论坛帖子,但我不知道如何完成这个操作,只是想问问别人。

<div id="stock_quote_iex">
    <form id="get_stock_quote_form" onsubmit="get_stock_quote()" method="GET">
        <p>Enter Symbol and API Key</p>
        <input id="symbol" type="text" placeholder="Symbol">
        <input id="api_key" type="text" placeholder="API_KEY">
        <input type="submit" value="Submit">
    </form>
</div>

<script language="javascript" type="text/javascript">
    function get_stock_quote()
    {
        var form = document.getElementById('get_stock_quote_form')
        var symbol = document.getElementById('symbol').value
        var api_key = document.getElementById('api_key').value
        var action_src = "https://sandbox.iexapis.com/stable/stock/" + symbol + "/quote?token=" + api_key

        form.action = action_src;
    }
</script>

Example inputs:
symbol = NFLX
API_KEY = pk_s0m3rand0mphra5e

Expected output:
https://sandbox.iexapis.com/stable/stock/NFLX/quote?token=pk_s0m3rand0mphra5e

Actual output:
https://sandbox.iexapis.com/stable/stock/NFLX/quote?