Custom code works on all paid plans (except for the archived Creator plan). With a Free plan, you can see working code in Preview mode but not in a published project.
First, find the post you want to embed, click on the three dots in the top-right corner and select '</> Embed post'. Then copy the final code and paste it into the Code widget.
Click on 'Set customization options' to personalize how the embedded post will look.
Protected posts cannot be embedded.