In texts with a more complex structure (non-fiction books, manuals, reference books, etc.), the text can sometimes be alternated with "read more" sections. For now, StreetLib Write doesn't provide text frames. We thus advise you to use one of following solutions (you didn't think we'd let you with no solution, did you?):
1) If the box is mainly filled with graphic content (images, graphs, etc.), the easiest solution is to treat them as images. This way, you can transform your box into an image and include it in your StreetLib Write text.
2) If the box mostly contains text, an alternative you can use is tables. You can, for instance, create a one unique cell table and define height and width according to your box's needs.
3) Finally, you could add a <div> with a personalized frame directly into the source code via StreetLib Write.