The context you are trying to use the function expression in expects the function keyword to start a function declaration.
A function declaration must have a name.
There is no point in putting a function there anyway: You never call it.
The value of an onclick attribute is the function body.
If you were going to go down this route then you code would look like this:
onclick="event.preventDefault(); document.getElementById('LogOut-form').submit();"
But don't do that!
Intrinsic event attributes are horrible for a variety of reasons. Bind your event handlers with JavaScript.
<a href="#">LogOut</a>
document.querySelector("a").addEventListener("click", function(event){
event.preventDefault();
document.getElementById('LogOut-form').submit();
});
But don't do that either!
You have a link to the top of the page which you are then "enhancing" with JavaScript to submit a form.
If the JavaScript fails, it doesn't make any sense to link to the top of the page.
Before applying JavaScript: Pick the HTML with the most appropriate semantics and default functionality.
In this case, that means use a submit button.
<button>LogOut</button>
If you don't like the way it looks, apply CSS to style it the way you want.
button {
text-decoration: underline;
color: blue;
background: none;
border: none;
padding: 0;
display: inline;
}
<button>LogOut</button>