<form action="https://www.paypal.com/cgi-bin/webscr" method="post" class="pp-form">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="[email protected]">
<input type="hidden" name="item_name" value="Doar">
<input type="hidden" name="return" value="https://untura.com/"> <!-- Change this for your thank you page -->
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="BRL">

Doar via Paypal:<br>

<input type="text" name="amount" value="" class="pp-amount"><br>

<input type="submit" name="PaypalPayment" value="Doar" class="pp-button"><br>

</form>

Aplicar o código acima e basta estilizar cada campo com CSS.

Fonte: https://allwebco-templates.com/support/S_paypal_custom_donation.htm