You can add Sagans to a website you edit in Squarespace by taking your Sagan's iframe code and pasting it into an Embed Element within your Squarespace editor.
1. To do this, first click one of Squarespace's Insert Points and choose 'Code' from the Block menu
2. Replace the default text with your Sagan's iframe code:
3. Click 'Apply'
For more info on adding Code Blocks to your Squarespace, see Squarespace's support article: https://support.squarespace.com/hc/en-us/articles/206543167