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There are many options as you can have XOOPS open on a default module...
So you could either have a module someone has made that fits, or just add a basic html submit button to a Smarty page and make your own module....that could then question if they wish to proceed...to enter
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Someone may have had to do this and have another solution for this though already...........
As also unsure if you could just add an index.html first, which auto opens before a index.php....yet then on submit open the php and stays with it....