Eventually I have found the way to do it and put it down here, as I have not found this advice anywhere.
First of all, you may find the External Mode feature under Hardware / Hardware Setting, under the Hardware Implementation tab on the left, under Hardware board settings / Target hardware resources. If you click on External mode, then you find the Communication interface, which is either serial or XCP over serial. The important is to set the Serial port to the one, to which the board is connected.
Then, instead of the Build, Deploy and Start button, press the Monitor & Tune button, which will flash the code to the board and start monitoring variables.
To see them, you have to open the Data Inspector window and may have to click the variables.
Luckily, it is nicely done, only badly documented, but YouTube helps.