I need to set up PayPal IPN masspay but I am having problems

This is my first time to set up a PayPal masspay. I don't know what variables should I put in my HTML form.

Here is the form:

<form action="https://www.paypal.com/cgi-bin/webscr" method="POST" name="_masspay">
    <input type="hidden" name="cmd" value="_masspay" />
    <input type="hidden" name="receiver_email_1" value="support@example.com" />
    <input type="hidden" name="receiver_email_2" value="paypal@example.com" />
    <input type="hidden" name="mc_currency_1" value="USD" />
    <input type="hidden" name="mc_currency_2" value="USD" />
    <input type="hidden" name="item_name_1" value="item name" /> 
    <input type="hidden" name="item_name_2" value="website fees" /> 
    <input type="hidden" name="amount_1" value="10.00" />
    <input type="hidden" name="amount_2" value="2.00" />
    <input type="hidden" name="custom" value="1" /> 
    <input type="hidden" name="return" value="https://www.example.com/buy/12" /> 
    <input type="hidden" name="cancel_return" value="https://www.example.com/buy/12" /> 
    <button type="submit">Pay With PayPal</button>
</form>

I don't know what I should have in the cmd field. This is my first time creating an HTML IPN form for masspay.

Can anyone help?