Right now K2 User must use K2 Login Module for submitting item in front end. I think It would be nice if K2 have menu link for submitting item so people don't have to activate K2 Login Module and use another login module. I prefer to use Community Builder login module because It's capability for horizontal orientation. Here is the example

piptek-tiui.vlexofree.com/K2/
user: demo1
pass: demo1

I use custom HTML module to make that button.

Thank you

Nice, this should work even if one is using the k2 login