Add custom HTML to wordpress widget - Tag script is not allowed

How can I add a custom HTML code to wordpress widget ? When I try to add <script> or <iframe>, I get an error: Tag script is not allowed / Tag iframe is not allowed.

Hi,

In the past, it was possible to add <script> or <iframe> tags to a widget, even in case of free accounts. But after a Wordpress update, using of some tags is not allowed on Free, Personal and Premium Plan, because of security reasons.

You can find out more at: https://wordpress.com/support/code/

To use <script> or <iframe> tags in widgets, you need to have a Business Plan with plugins installed.