Hi Jennifer,
We should put the script on marketing page.
From my using experience, I always put it after end tag of body element.(And the script should be nested in script tag.)

--- How do I proceed when we host it externally via iframe?
I am not so clear about what does "host it externally via iframe" mean.
Could you share me more details about it?
As per my understanding, do you mean the form is being hosted on an external website such as Wordpress?
If so, as far as I know, some WP plugins might create an iframe element on page and custom elements would be put to the iframe.
To handle such situation, we could:
> If custom elements were actually put to iframe, then we need to paste form embedding script to web page directly without using plugin.
> If iframe is actually another web page, then we should put the script to that corresponding page.
In a word, elements in iframe couldn't be accessed.(A more accurate statement is that we couldn't inject javascript into iframe.)
Please kindly point out if I had misunderstood towards your question.
Regards,
Clofly