What's the easiest way to create a data entry form for a Quickbase application that I can embed on my website?

The most common setup for these web forms is to create a public access role in your app, to open your app up to Everyone on the Internet. You can set up that role to allow only what it necessary for your form (so you'll likely just need to allow EOTI users to add new records to that specific table).

You can read more on EOTI here: https://help.quickbase.com/user-assistance/share_with_everyone_on_internet.html