what i would do is to locate the name of the script you send your password to for hotmail/yahoo/etc. (it will be in the page source somewhere), and look for any hidden variables that are passed as well.
you could do it in two pages easily, by selecting whichever account you want on one page, and a second submit button on the other that has the hidden variables filled in by a small script
<input type="hidden" name="username" value="
<?
if( pageonehidden==1){
$hotmailpassword=abc;
echo $hotmailpassword
}
?>
">
then submit to hotmail or yahoo etc.
or you could set some variables in the url
www.somewhere.com/abc.php?username=user&password=mypass
but you can already see the BIG security problem there.
I still suggest the hard way - much safer.
Stuart