A batch file to launch the game or auto inject using xenos (https://github.com/DarthTon/Xenos) injector the GTA online patch: https://github.com/QuickNET-Tech/GTAO_Booster_PoC.

launch_gtao_patch.bat

:: Launch GTA Online Patch v1.2
::   This batch file will automatically launch GTAV if not started, look for the process and inject using Xenos
::   You should probably at minimum change the GTADIR parameter below to where your GTA5.exe is
::   It will run Xenos64 in the folder the bat file is ran from, for the profile "OnlinePatch.xpr64"

:: Links
::   Xenos 					- https://github.com/DarthTon/Xenos
::   Online Patch Booster 	- https://github.com/QuickNET-Tech/GTAO_Booster_PoC
@echo off
title Launch GTA Online Patch


:: If you want to auto launch as admin, uncomment this: (Relaunches batch as admin)
::if not "%1"=="am_admin" (powershell start -verb runas '%0' am_admin & exit /b)

::  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
:: 	!!! These variables may need to be changed !!!
::  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

:: 	The directory where PlayGTAV.exe and GTAV.exe are located
Set GTADir="S:\Grand Theft Auto V"
:: 	Is the game STEAM or WAREHOUSE? TRUE for STEAM, blank for WAREHOUSE 
Set IsSteamVersion=true
:: 	How long until after process detected to inject? (in seconds)
::  1 second allows for patch to skip r* credits, > 2 will be too late. 
Set WaitToInject=1
::  The name of the injector profile with the extension 
Set ProfileName=OnlinePatch.xpr64
:: 	The directory to find the executable, defaults to location of this .bat file.
set ExecutableDir="%cd%"

:: Executables (You don't need to change these!)
Set Process=GTA5.exe
Set RockstarLauncher=Launcher.exe
Set ExecutableFileName=Xenos64.exe

:: If GTA5.exe not running, starts via PlayGTAV.exe (May not work on Steam version)
cd /D %GTADir%
tasklist /NH /FI "imagename eq %Process%" 2>nul |find /i "%Process%" >nul
If errorlevel 1 (
	echo Game is not running, starting. STEAM: %IsSteamVersion%
	If defined IsSteamVersion (
		start "" "steam://run/271590//-goStraightToMP -StraightIntoFreemode/"
	) else start PlayGTAV.exe -goStraightToMP -StraightIntoFreemode 
)
 
:: Starts looking for the process
echo Looking for %Process%...
:findGTA
tasklist /NH /FI "imagename eq %Process%" 2>nul |find /i "%Process%" >nul
If not errorlevel 1 (
	:: If process is found, inject.
	echo GTAV Found, Running %ExecutableFileName% for Profile %ProfileName%
	cd /D %ExecutableDir%
	timeout /t %WaitToInject%
    echo Running executable
	start /WAIT %ExecutableFileName% --run %ProfileName%
	echo Executable ran, will automatically close when game exits.
	goto waitForGameExit
) else (
	:: Process still not found, continue loop after 1s
	timeout /t 1 /nobreak > nul
	goto findGTA
	
)

:waitForGameExit
tasklist /NH /FI "imagename eq %Process%" 2>nul |find /i "%Process%" >nul
If errorlevel 1 (
	echo Detected game has closed, exiting...
	timeout /T 2 /nobreak > nul
	echo Closing social club launcher
	wmic process where name="%RockstarLauncher%" call terminate
) else (
	timeout /t 6 /nobreak > nul
	goto waitForGameExit
)

v1.2 actually fixes steam support (Put a value for IsSteamVersion, leave blank if not)
In addition, there is a line that can automatically relaunch script with admin permissions, uncomment it if you need it.