Hi @gaatec, in general this code (JavaScript I am assuming) could be added to one of the Twig files of your current theme. In order to not get this possibly removed when a theme update happens you should first make an inherited theme: https://learn.getgrav.org/themes/customization#theme-inheritance
If you provide the name of the theme you are using I might be able to give you more specific info.
Are you trying to add a contact form within an iframe? If so, you should be able to do this by editing the template file contact.html.twig within the folder user/themes/clean-blog/templates/.
In the above file look for this:
<div class="col-lg-8 col-lg-offset-2 col-md-10 col-md-offset-1">
{{ page.content }}
{% include "forms/form.html.twig" with { forms: "contact-form" } %}
</div>
And try to replace the code between the two div tags with your SALESmanago iframe code: