Help needed with systemd script

[Mattia Verga]

Hello,
I'm trying to give a user access rights to X with xhost command.
I've created a script named '/usr/bin/boincxhost' (chmod 555) like this:

/#!/bin/sh//
//xhost +SI:localuser:boinc &> /dev/null//
//exit 0/

If I run it from command line it works fine.
But I want to run from a systemd unit at startup. So I added this line 
to the unit file:

/ExecStartPre=/usr/bin/boincxhost/

The service starts without errors, but I cannot find the user to have 
rights for accessing X. Maybe someone can tell me where I'm wrong in the 
procedure? Or is there any other way to give that user access to X? I 
already tried to add the user to the 'video' group, but Boinc refuse to 
see the GPU available.
Thanks for any hint.

Mattia
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