tinymce-docs-4x/_includes/configuration/init-instance-callback.md

init_instance_callback

This option should contain a function name to be executed each time a editor instance is initialized. The format of this function is initInstance(editor) where editor is the editor instance object reference.

Type: JavaScript Function

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  init_instance_callback : function(editor) {
    console.log("Editor: " + editor.id + " is now initialized.");
  }
});

Also see the the setup callback option it enabled you to bind events before the editor instance is initialized.