@Vasco_Ramalho - I am pretty sure that this is not possible and it is a feature only available via the designer.
A possible workaround is to create a text field to hold the content. You can use it in a template but just have to decode it since Webflow encodes HTML on render. If you needed the code rendered inside the contents of a RTF you would need a target to replace in your code. Something like decode the element, look for a string that was used as a target, then replace the target with the decoded code block.