This question was posted in response to the following article: http://help.adobe.com/en_US/indesign/cs/using/WSB152EA6F-30D9-417c-9E16-A2C492C04892.html
How to open the hyperlinked url in a new tab of the same browser window in which the pdf file is opened up already?
It can't be done in InDesign but can be accomplished in Acrobat. Do a Google search on "Acrobat open link in new tab"
Here is a sample thread with a couple answers: