On version 6 onwards i can do it by putting the script on the header section of the HTML modules settings page.
Script is like, i am calling the below method on click event of the hyperlink.
dnnModal.show(MyUrl + '?popUp=true', false, 550, 950, false);

Which opens a popup on the click of the link present on the HTML module.

But this thing is not working on versions prior to 6. Looks like dnnModal is not supporting.

Any Idea???

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Thanks,
Sunil Kumar [ DNN Developer, Mindfire Solutions,India ]