Personally, I steer clear of XPath... and here's why... finding something via XPath will return something... but that something might not be a UI object that can be operated on with the standard wrapper actions/methods from TestComplete. Our pal AlexKaras can give a better explanation.
The primary method (preferred and recommended) for object identification in TestComplete is via the NameMapping engine which builds an object hierarchy structure where objects are identified by a) what they are a child object of and b) a set of object properties used as identifciation criteria. So, your example would look, in the TestComplete programming, like
Aliases.browser.pageMyPage.buttonLogoff.Click()
See https://support.smartbear.com/testcomplete/docs/testing-with/object-identification/name-mapping/about.html and all related articles.