TGYoshi says:
You can alternatively use the following, which will forever work and keep compatibility with older maps as well:

... says:
@find /c /i "exever=""3." %1 > NUL

This post by Yoshi is really important, as different map versions exist on the site, this is the approach to use.

still not working
@Space, I know where I install my games

The following is written in my .cmd


ECHO OFF
@find /c /i "exever=""3." %1 > NUL
if %ERRORLEVEL% EQU 0 (
start C:\Program Files (x86)\ManiaPlanet\ManiaPlanet.exe

/useexedir /singleinst /file=%1
) else (
start C:\Program Files (x86)\TmUnitedForever\TmForever.exe

/useexedir /singleinst /file=%1
)

try putting paths that contain spaces in quotes...

start "C:\Program Files (x86)\ManiaPlanet\ManiaPlanet.exe"

you really have /useexedirblabla on new line?

der-bozz says:
still not working
@Space, I know where I install my games

The following is written in my .cmd


ECHO OFF
@find /c /i "exever=""3." %1 > NUL
if %ERRORLEVEL% EQU 0 (
start C:\Program Files (x86)\ManiaPlanet\ManiaPlanet.exe

/useexedir /singleinst /file=%1
) else (
start C:\Program Files (x86)\TmUnitedForever\TmForever.exe

/useexedir /singleinst /file=%1
)

You can test the script in a command shell for errors, giving the wanted Gbx file as parameter, e.g.
"C:\Program Files (x86)\ManiaPlanet\RunManiaPlanet.cmd" "E:\Downloads\XXX.Map.Gbx"
so you see error messages printed in the command shell before it disappears.