Command in steam prefix

On linux steam runs games with proton/wine, which is great, but sometimes one wishes to tun another command within that prefix, for example a mod manager.

Here is a solution I have found to do just that:

You can see the command that steam runs for a program in the log file:
~/.steam/steam/logs/content_log.txt

Copy the command into the launch options (right click on game, properties).

The end of the command is the exe that is run, you can change that.

Before closing, add this to the end:
& echo %command%

This stops what you added being appended to the usual command and instead puts it first and the command that it intended to run is just echoed... into oblivion.