Hello Fabián,
I’m afraid neither the online compiler nor the Mbed Studio support custom commands. Nor does the mbed-cli tool let you do it directly (run mbed compile -h to check for all available options). That’s why to run more commands in sequence (pre-build + build + post-build) I usually create a batch file. For example:
build_my_program.bat:
git describe --abbrev=6 --dirty --always --tags
mbed compile ...
Best regards, Zoltan