Here is the basic idea of pre-populating a form and most forms are pretty much the same with the exception of having to use your companies site path info. The fields shown in boldface are the pre-population fields. Give it a NAME and enter your referral id as the VALUE.
<FORM ACTION="http://www.yoursite path.com" METHOD="POST">
<INPUT TYPE="hidden" NAME="cmd" VALUE="_xclick">
<INPUT TYPE="hidden" NAME="business" VALUE="
[email protected]">
<INPUT TYPE="hidden" NAME="undefined_quantity" VALUE="1">
<INPUT TYPE="hidden" NAME="item_name" VALUE="hat">
<INPUT TYPE="hidden" NAME="item_number" VALUE="123">
<INPUT TYPE="hidden" NAME="amount" VALUE="15.00">
<INPUT TYPE="hidden" NAME="shipping" VALUE="1.00">
<INPUT TYPE="hidden" NAME="shipping2" VALUE="0.50">
<INPUT TYPE="hidden" NAME="currency_code" VALUE="USD">
<INPUT TYPE="hidden" NAME="
first_name" VALUE="
John">
<INPUT TYPE="hidden" NAME="
last_name" VALUE="
Doe">
<INPUT TYPE="hidden" NAME="address1" VALUE="9 Elm Street">
<INPUT TYPE="hidden" NAME="address2" VALUE="Apt 5">
<INPUT TYPE="hidden" NAME="city" VALUE="Berwyn">
<INPUT TYPE="hidden" NAME="state" VALUE="PA">
<INPUT TYPE="hidden" NAME="zip" VALUE="19312">
<INPUT TYPE="hidden" NAME="lc" VALUE="US">
<INPUT TYPE="hidden" NAME="email" VALUE="
[email protected]">
<INPUT TYPE="hidden" NAME="night_phone_a" VALUE="610">
<INPUT TYPE="hidden" NAME="night_phone_b" VALUE="555">
<INPUT TYPE="hidden" NAME="night_phone_c" VALUE="1234">
<INPUT TYPE="image" NAME="submit" BORDER="0" SRC="http://www.paypal.com/en_US/i/btn/btn_buynow_LG.gif" ALT="PayPal - The safer, easier way to pay online">
<img alt="" border="" width="1" height="1" src="https://www.paypal.com/en_US/i/scr/pixel.gif">
</FORM>