Script to join checkbox values into string before submitting

I have a paypal checkout form where I need to join together multiple chosen checkbox values (name="cb1") into a single string so it can be submitted as a hidden input (name="os1") value. I'm going round in circles trying to figure this out. I know very little JS! Can somebody help please?

<div id="buyform">
        <form target="paypal" id="ppform" name="ppform" action="http://ift.tt/rDmnwQ" method="post" >
        <input type="hidden" name="cmd" value="_cart">
        <input type="hidden" name="business" value="myPPbusinessid">
        <input type="hidden" name="lc" value="GB">
        <input type="hidden" name="item_name" value="Form Title">
        <input type="hidden" name="button_subtype" value="products">
        <input type="hidden" name="no_note" value="0">
        <input type="hidden" name="no_shipping" value="2">
        <input type="hidden" name="currency_code" value="GBP">
        <input type="hidden" name="add" value="1">
        <input type="hidden" name="bn" value="PP-ShopCartBF:btn_cart_LG.gif:NonHosted">
        <div><input type="hidden" name="on0" value="Options"><h5>Options</h5></div>
        <div>
            <select name="os0" id="os0" required>
                        <option value="Chosen Product">Any quantity £10.00 GBP</option>
                </select>
        </div>
        <input type="hidden" name="on1" value="Colours">
        <div>
        <input type="checkbox" name="cb1" value="Red" id="os10" class="thecbox"/><label for="os10"></label>
        <input type="checkbox" name="cb1" value="Green" id="os11" class="thecbox"/><label for="os11"></label>
        <input type="checkbox" name="cb1" value="Orange" id="os12" class="thecbox"/><label for="os12"></label>
        <input type="checkbox" name="cb1" value="Purple" id="os13" class="thecbox"/><label for="os13"></label>
        <input type="checkbox" name="cb1" value="Yellow" id="os14" class="thecbox"/><label for="os14"></label>
        <input type="checkbox" name="cb1" value="Black" id="os15" class="thecbox"/><label for="os15"></label>
        <input type="checkbox" name="cb1" value="Blue" id="os16" class="thecbox"/><label for="os16"></label>

        <input type="hidden" name="os1" value="EACH, CHECKED, VALUE, HERE">
            
        </div>
        <input type="hidden" name="currency_code" value="GBP">
        <input type="hidden" name="option_select0" value="Options">
        <input type="hidden" name="option_amount0" value="10.00">
        <input type="hidden" name="option_index" value="0">
        <input type="hidden" name="currency_code" value="GBP">
        <input type="submit" border="0" name="submit" id="submit" value="Purchase" class="buynow mb0"></form>
        </div>