Hi Josh,
Emlid Caster doesn’t provide corrections per se. It works more like a ‘pipeline’ to pass the corrections between your own base and rover via the internet.
What you need to place the base via NTRIP is NTRIP Service (CORS). Such services place several receivers as bases and provide access to the data from them. Please note that they may take a fee.
Here is also an article to learn more about the difference between NTRIP Caster and NTRIP Service.