You cannot currently create a document via the API and leave the smart content block in a “pending selection” mode (i.e., so the user can later choose from the pre-selected content in the UI) in the exact same way as you can in the Editor. When using the API, you must specify which content library item(s) to insert into each smart content block at the time of document creation, or make the block optional (which means it can be empty in the generated document). The API does not support leaving the smart content block in the same “interactive, user-selectable” state as the web UI—once a document is created through the API, the block must be resolved to specific content or left out per your selection rules.