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 these boxes management (or text frames). We thus advise you to use one of these solutions (you didn't think we'd let you with no solution, did you?):
1) If the box in question is mainly filled with graphic content (images, graphs, etc.), the easiest solution is to deal with these frames as images. You can change your box into an image and include it in your StreetLib Write text this way.
2) In the case of a box mostly containing 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 act directly on the source code in StreetLib Write, inserting a <div> with a personalized frame.