> The code also gets wiped on this forum so I cannot show what it looks like.
You have to correctly escape it using for example backticks. See the markdown instructions below the form field.
> NFT stands for non fungible token by the way. The code I want to use is a embed code from opensea.io where you can make those things
Ah, of course heard of that but have no idea how those look like. So I cannot help a lot.
Generally output is of course sanitized so nothing sneaks in. It might be that you need to do some more custom stuff here like editing the theme used.
> How do I make a config file for tinymce?
https://www.zenphoto.org/news/how-to-create-custom-tinymce-configurations/