Microbit - Control an LED that is powered externally
Hello Tyris and welcome!

If I understood right, you just want simply on/off connection to your led controlled by micro:bit. 

You need to use transistor or mosfet etc etc, which will work as a driver and handle higher currents which led requires.

You can use use p-channel mosfet for example. This kind of Mosfet is little bit overkill here, but great to understand concept.

Here is source to understand what you are trying to accomplish:

https://www.electronics-tutorials.ws/transistor/tran_7.html

Attach your led as in image and calculate needed resistor. 

some help from here to understand current limiting 

https://www.sparkfun.com/tutorials/219)

Those are just first links which appeared from google but there is tons of articles about this subject if those are not enough which I provided here.