You would need to the PayPal HTML variables. There's ton of info here. The variable you need to use is "amount". Note, you cannot include any currency symbols as the Checkout Screen will error - for examle, if you enter $5.00, it won't work however 5.00 will. Below is a basic Donation example that demonstrates how to enter an amount on your web page. Just replace the "business" variable value with your PayPal email address or your Merchant ID. Copy the code to a blank page, you can even use Notepad, save the file as Test.html and then you can open it with your browser, enter an amount and there you go.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title></title>
</head>
<body>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="your email address">
<input type="hidden" name="item_name" value="Special Event">
<input type="hidden" name="item_number" value="SE-001">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return" value="http://www.mysite.com/thankyou.html">
<input type="hidden" name="cancel_return" value="http://www.mysite.com/cancel.html">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="tax" value="0">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-DonationsBF">
Please enter your donation amount:<br>
<input type="text" name="amount" value="">
<br><br>
<input type="image" src="https://www.paypal.com/en_US/i/btn/btn_donate_SM.gif" border="0"
name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
</body>
</html>
