It's very easy. Just go to the cPanel/Webmail login page(s) and grab the code.
Here's the example for cPanel login:
HTML:
<form action="/login/" method="POST">
<table>
<tr><td>Username</td>
<td><input type="text" name="user" size="16"></td></tr>
<tr><td>Password</td><td><input type="password" name="pass" size="16">
</td></tr><tr><td colspan="2"><div align="right">
<input type="submit" value="Login">
</div></td></tr>
</table></form>
Here's the example for the Webmail login:
HTML:
<form action="/login/" method="POST">
<table>
<tr><td>Username</td>
<td><input type="text" name="user" size="16"></td></tr>
<tr><td>Password</td><td><input type="password" name="pass" size="16">
</td></tr><tr><td colspan="2"><div align="right">
<input type="submit" value="Login">
</div></td></tr>
<tr><td colspan="2"><div align="left">
<b>Note: Your Username is usually your email address.</b></div></td></tr>
</table></form>
I don't understand what you're talking about with flash and asp and all of that.
Take a look at the link to my site in my sig and on the left side, you'll see a webmail login. That login is for Roundcube, but you should be able to do something like that if you implement the code correctly.
Good luck!