Handle the CustomizeFunctions event and add new functions in the following way:

   void expressionEditor1_CustomizeFunctions(Dictionary<string, AdvancedKeywordInfo> functions)
   {
       functions.Add(
           "customfunction1", // dictionary key for hashing (lowercase!)
           new AdvancedKeywordInfo(
           // Function display name
               "CustomFunction1",
           // Function template that will be inserted into the text editor cursor position. 
           // May contain %CURSOR% macro to specify new cursor position.
               "CustomFunction1(%CURSOR%)",
           // Function description (displayed in tooltips). Use \n character to split lines up.
        "CustomFunction1(param1, param2)\nFunction description.\nBla Bla..."
        ));

       functions.Add(
           "customfunction2",
           new AdvancedKeywordInfo(
               "CustomFunction2",
               "CustomFunction2(%CURSOR%)",
               "CustomFunction2(param1, param2)\nFunction description.\nBla Bla..."
        ));
   }