So we need to break down the embed code in a couple of steps. So first you have the embed code they gave you. This is an example embed from the link:
src link & code below that and add it to the Custom JS like so (make sure to swap out the store id with your own in the src link and code):
// src: https://app.ecwid.com/script.js?1003&data_platform=code
Once that is added you can save the changes. After this you can go to the post you want your embed to be and add the top HTML code in like so:
# Fake Title
Once you save this should do the trick.
Let me know if that works for you. Might have to do some other things to get it working but should be doable.