Paypal redirect for subscription but parameter disappear

I am trying to post a form to PayPal for subscription.
I created a form and tried following ways to add parameters src, a3, p3 and t3 as requested by Paypal:
1. add hidden fields in Designer
2. change form type to "custom code" and copy + paste following
<input type="hidden" name="src" value="1">
<input type="hidden" name="a3" value="10.00">
<input type="hidden" name="p3" value="1">
<input type="hidden" name="t3" value="M">
3. add something like ts=M and p3=1 in "Extra Param" of "setup -> PayPal redirect "

But these parameters either totally disappeared or the value missed out. Is that I need a commercial license to proceed?

Thanks.

Hi Arnold,

The "extra param" field will map PayPal params to form fields names, so if PayPal needs the param named "a3" for example, and this param's value is in your field name "a3" then you only need to write:

a3=a3

You could also simply add the hidden fields code to a "custom" field under the "designer" section, without the need to change the form to "custom code", because now you will not be able to do new changes in the designer!

Regards,
Max