Mailto function in flash is not much different than Mailto function in HTML. In HTML this function is applied on links, whereas in Flash it is applied on buttons.

<a href="mailto:user@email.com ?subject=Feedback">click here </a>

this is how it looks in HTML, to test how it works click here

And this is how it looks in Flash:

on (release) {
         getURL("mailto:user@email.com?subject=Feedback");
}

Just create a button in flash, click on your button and open the Actions panel and copy the above code inside Actions panel. Change the email and subject to your own.

Here is how it should look and work after you have published it and implemented inside the HTML page.