the function is "enterword()", its a usual javascript function, the $.ajax is part of the jQuery UI, which needs to be referenced.
to call the function from "c#" (i am guessing that you mean "ASP.NET") you simply need to write "enterword();" to the appropriate attribute.
Here is an example:
<asp:button Id="myId" Text="click me" onClientClick="enterword(); return false;" />
Here is an HTML Example:
<input type="Button" value="click me" id="myId" onclick="enterword(); return false;" />
the "return false;" is mostly obsolete but i still use it, it overrides the defaultbehaviour of the browser after the javascript is executed.