tinymce-docs-4x/_includes/configuration/forced-root-block-attrs.md

forced_root_block_attrs

This option enables you specify attributes for the forced_root_block.

Type: Object

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  forced_root_block_attrs: {
    "class": "myclass",
    "data-something": "my data"
  }
});