I have an ASP.NET button. If the user clicks this button, then it calls `mailto`, which opens the outlook mail window. I have done this by adding the following line into the ASP.net button control tag.

`window.open('mailto: abc def<abc.def@ex.com>?subject= exSub &body= exBody');`

Now I want to set the body text (in the above example it is exBody) dynamically in my code behind method. How can I do that?