Sorry, just tested quickly in my PDI, there is no need to create a custom True/False field and change the menuTemplate code, simply put the following in the URL target field

_blank

IMPORTANT: check if you have any rule or filter in your browser that will BLOCK the new tab working or simply try using a different browser.

It works like a charm in my PDI, if you're in trouble, check my settings of the Menu Item:

This is my test using Chrome browser:

If I have answered your question, please mark my response as correct and/or helpful.

Thank you very much

Cheers
Alberto