To embed a site into your formative, you can either:
Choose the embed content option:
Or click the "+" icon within a question, and select "Embed":
How to embed a simulation from PHET:
Go to https://phet.colorado.edu and click on "Simulations"
When you find a simulation you like, click on "</> Embed" and copy the code
Paste the code into the Embed field in your formative!
First,
Complete this simulation:
Then,
Screenshot your result and upload it here: