To add a custom embed code or raw HTML, you will first need to set up a field specifically for this. Then you will need to add code to your website application to handle this new field.
You have two options, using a Key Text field or a Rich Text field.
We recommend you using a customized Rich Text field specifically for your custom code. You will need to configure the field to "Allow multiple paragraphs" and set the field only to allow preformatted text.
If you use the Key Text field, you can paste the code you need into it. Then it will output as plain text, which you can inject straight into your code. This option will appear as one big string in the Prismic interface.
When using this option, your code might also be encoded when you save the document. Using this method, you will likely need to unencode the content before adding it to your pages.
The next step is to update your website code to handle this new field. You will need to output this field as plain text straight into your template.
If you aren't a developer, then you will likely need help from your dev team for this step.