Design the Adaptive Card and post it via Microsoft Graph

Question

I'm trying to send the adaptive card using the Microsoft Graph API. While using the code example from Docs, everything works perfectly. But I really want to play with the design to add more relevant and complicated content. To design the card, I'm using the Adaptive card Designer.

As I understood, I need to transfer Json content I received after designing the card into the string format, and then past it instead of existing "content" value from the Microsoft Learn example. Am I right? But in this case, the message is sent with no content. What I'm doing wrong?