How to call global function in expression editor

Summary:

I have a requirement :: one of the command must be display when the global function returns true and disable when the method returns false . In the command I need to call global function in expression editor using expression builder. Can any one help me to fix the issue .

Content (required):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):