Where do I find the exact ‘carrier_code’ I need to send to Magento2 to add my shipment through API

I want to add using the /rest/V1/shipment/track API. My shipment. We work with 3 shippers:
DHL
UPS
PostNL.

Now I need to know the carrier_code in order to add the correct information for the tracking link to work. However, I’ve googled for a long time and found no such thing as a list of exact codes need to send for each service. Nor does it say in my shop backend.

Is the title field under shipment free to fill in yourself?