Hi Stephani,
As per my understanding, you can take fields contained in success callback data for personalized content.
Yes, innerHTML is a property of DOM element on HTML page, it's can be used to element display customization, to say "Hello, firstname", simply change the content string at right of innerHTML like: "Hello, " + firstname(from data.Data["firstname"]) ...
We need some basic JavaScript skills to make our personalization.
Here is how did I add the context to the body content, it will be generated at top of marketing page.
My personlization is an image will be shown to contact when its city field equals some value,

Result:

Or you could take callback data to populate fields.
Regards,
Clofly