Hi @Montero , thanks for posting on this community.
On this article: https://help.brevo.com/hc/en-us/articles/360000486460-How-do-I-create-edit-the-plain-text-version-of-HTML-emails
I see that there is an option called « View the HTML code » at the Design step of the campaign (see the screenshot inside the article). Could this be helpful to you?