Can you provide a link to your website with a form styled this way as a reference?
I agree w/ Sanford, this should be possible with some CSS and maybe a little JS to fill in the gaps. I'd be happy to have a look at how this is setup on website for context and think about how that might translate into a Marketo form.
In terms of how/where it's implemented (on the form vs. on the website) I'll normally recommend setting up on the website-end.
In a nutshell:
- If you add it to the form, it'll travel with the form wherever it goes (on a Marketo LP, on the website, etc). MOST times this creates some kind of styling conflict b/c your Marketo LP templates and your website templates will often have different sets of form styling to work around.
- If you add it to the website, the styles will translate to ANY form that you put on a page rather than "just this one form". This keeps you from having to copy/paste a bunch of CSS from form-to-form in Marketo as well.
If you'll need to use any JS to support this, I'd recommend adding that on the website end as well. While it is technically possible to add script to a Marketo form, it's not a best practice in my experience.