Hi @david53
Please, follow the next steps:
- Go to Online Store → Edit Code
- Appears a new page with a left menu and many folders like: layout, sections, snippets, etc.
- Find the folder layout and open the theme.liquid file
- From here you need follow the instructions that you put in your question.
- Save changes.
- Go to your site and the iframe must appears.
Note: These instructions make the iframe appear on all screens of your site. If you need something more customized, feel free to contact me.
Regards.