LEDs have little internal resistance. They need a limiting resistor, otherwise they burn up, essentially instantly.
The value of the resistor depends on the current required by the LED, which should be specified by the manufacturer.
This calculator can help.
https://www.pcboard.ca/led-dropping-resistor-calculator
Use "14" for volts and "1.5" for voltage drop across LED (if not specified by the manufacturer), it can be 1.5 to 3.3 but you should be safe to assume 1.5.
If you buy an LED that says "for 12 volt operation" it already has a dropping resistor in it's leads or it's housing.