You can see how to add a header in this video
(Internal note: the styling of the header will be taken automatically from the website)
Please see this video how to add a divider (line).
The text you see in the video to add is the following: <hr>
You can't, unfortunately, add a look-up field in a form and retain the function of searching through the drop-down menu. You have to add it as a Select field (without the search function) or re-create the custom field as a text input field (but you won't be able to easily segment based on this custom field)