tinymce-docs-4x/_includes/configuration/body-id.md

body_id

This option enables you to specify an id for the body of each editor instance. This id can then be used to do TinyMCE specific overrides in your content_css.

Type: String

Example:

This will add the same id to all editors that gets created by the init call.

tinymce.init({
  selector: "textarea",  // change this value according to your html
  body_id: "my_id"
});

This will set specific ids on the bodies of specific editors.

tinymce.init({
  selector: "textarea",  // change this value according to your html
  body_id: "elm1=my_id, elm2=my_id2"
});