I'm using YahooMail and wish to have defined as my default email program ...
Well, the basic problem here is that YahooMail is not a program. It's really just a website and you view it through your web browser. And, web browsers are not "email programs", they are programs used to display web pages.
Examples of true email programs are Outlook Express, Outlook (usually bundled with Microsoft Office), Windows Mail (comes with Windows Vista), and Windows Live Mail. And then, there are some non-Microsoft email programs such as Thunderbird (from Mozilla). Again, these are true email programs, not a web browser.
So, if there's a way to get YahooMail to work the way you want, I'm not aware of it. I believe you'll need to copy and paste to forward a page or link. Just copy the link to the web page into your email message.