You'll need to post more info - like the Script you are using for example ...
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That said, here's a guess at what might help you :
%RND := 1D20
IF %RND < 4 THEN
$DEST := @P100
So, the above rolls a d20, if the result is less than 4 (a 15% Chance), then it redirects the Link to page 100. If the above is not true, then it just goes to the Original Link Page.