Mathematica can add autocompletions to functions very nicely. There are ways to do this you can find on the stack-exchange, but operationally they look like:

AddFunctionAutocompletions[myFunc, {{"arg1", "arg2", "arg3"}}];

And then I can test this like:

myFunc["ar

But how the front-end does this for Entity is beyond me and something I'd love to have. Consider this:

But the box form of this looks like:

ToBoxes@Entity["Country", "China"]

TemplateBox[{
  "\"China\"", 
  RowBox[{
     "Entity", "[", 
       RowBox[{"\"Country\"", ",", "\"China\""}], 
     "]"
    }], 
 "\"Entity[\\\"Country\\\", \\\"China\\\"]\"", 
 "\"country\""
 }, 
"Entity"
]

Some simple debugging lets me know that all the FE is doing is using the second argument to that TemplateBox to get this out, but how it does that is beyond me and how this is set in the kernel I don't know.

So...how do I do this myself? This would be a great feature to have for OOP.