Sadly, I can't find my new SIM card. Can't guarantee it wasn't just thrown in the box, but I never saw it. T-Mobile is shipping a new one. In the meantime, I'm about to put my old card in, but T-Mobile support site says this:
With the device off, open the SIM card tray and insert the SIM card.
If your device didn’t come with a SIM card, just move the SIM from your old device to your new one and you’re good to go, no additional steps required.
Here's a short video to guide you through the process.
Activate the SIM card. For the best 5G experience, we don't recommend using your previous SIM card.