You can't add the parameter '-file' to the program setup.
To run custom WADs, you have to use either ipxsetup or sersetup.
This programs can use the '-file' parameter.

Example for ipxsetup.exe:

ipxsetup.exe -nodes 2 -skill 4 -file testfile.wad

Example for sersetup.exe(modem game):

sersetup.exe -com1 -irq 4 -16550 -57600 -answer -skill 4 -file testfile.wad

You can also use Deathmanager (DM.EXE) to setup multiplayer games with custom WADs, it's essentially a retooled setup.exe network game interface.

I forgot about this... 😁 thank you leileilol.
There was an old frontend, okay there were many frontends, but I mean the D!-Zone frontend. I'm using this one for playing all my WADs 😁

Maybe you'll look for it, I think there was also a demo version of it.
I'll search for it.. think I got it somewhere.

Ah I found it! It was on the cd 'DOOM Developers Kit V1.1'...