Thank you flamemaster.
But i was talking about completely different thing . You're talking about snapping to curve, which is "C" shortcut but i was talking about how to make the curve closed and not only visually closed, but to have the word "closed" in attribute editor, which is usually periodic.

Anyway i started the same thread at cgtalk and i already got all my answers and i want to share them here as well. First of all it's possible to close only cubic curves and there 2 ways of doing it. One as i described in my first post by selecting Edit Point and making Detach Curves. And another one is making Edit Curves>Offset>Offset Curve - which makes the curve closed but it's easier to make detach command rather than making offset and then deleting the original and etc. etc.

But i needed a closed curve to apply Bevel Plus to two selected curves at once which i thought wasn't possible, but one guy helped me out solving this problem and it turned out to be a bug in Maya which is very funny. When you create curves in perspective view, you then need to rotate curves 90 degrees in X and then apply Bevel Plus to both of them and it will work, but if they're not rotated, then it won't work - the alternative way of doing that is to create curves in front view straight away.

So you don't need the curve to be closed (once again by saying closed i don't only mean to have start point and last point in one location but actually have it saying "closed" in attribute editor). But still i wanted to mention how to make it closed in case if someone needs it .

And here's the link - https://forums.cgsociety.org/showthread.php?t=386684 to thread at cgtalk about it in case if someone's interested but i already summed everything up here and the result is rotating curves 90 degrees in X and only then applying Bevel Plus (when we talk about 2 curves at once).

So i hope that helps.