Hi Chnama,
This setting is not located in this right-click menu. You will need to locate it in the Static Mesh Editor for the specific mesh you want.
You can see this Wiki page for steps to use this feature: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
Thank you!
Tim
I wanted to add that in later versions of the engine (4.15+) the ability to generate unique ids has moved into the Build Settings rollout. There you can specify the lightmap resolution, which channel you want to generate unique UVs from, where it should put those unique UVs etc. Just make sure you hit the Apply Changes button after you make your changes.
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