Add textbox instead of a dropdown selectbox?

I'm using LM Paypal with Donation. Is there a way to add a textbox instead of a list of amount to select from.

We want people to be able to just type in the amount they wish to donate and not have to select from a predefined list of value.

The code snippet i used:

<?php
if (function_exists('lm_paypal_donate')) {
lm_paypal_donate(array(5,array(10),15), 'GBP', 'donation to example.com', '', 'Donation') .'<br/>';
}
?>

This creates a list but we only want a textbox so users can enter their own amount to donate.

Thanks,