how to use sendmessage to call a unity function from html

The official doc says use SendMessage SendMessage ('MyGameObject', 'MyFunction', 'foobar'); But after I built a webgl and add SendMessage to the released index.html, I got "SendMessage is undefined". And I add this var u=GetUnity(); u.SendMessage ('MyGameObject', 'MyFunction', 'foobar'); then I got GetUnity is undefined How can I solve this problem