Add target="_blank" to the end of the URL
When linking in the MT Add Entry form, type the words you want to link, highlight them and click the URL button. Type in the URL, end it with double quote, then space and the target definition leaving off the last double quote, like this:
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http://www.somewebsite.com" target="_blank
MT will add the last double quote for you. And your link will open in a new window.