When the batch file runs, it's going to be relative to whatever directory that it's in. Given that, you need to make sure that when you call horizons.exe or istaria.exe that you first change directory to your Horizons install directory so that the script knows where to find it:
c:
cd "C:\Program Files\Artifact Entertainment\Horizons"
Be sure you reference the actual install location on your computer which is likely different than the above.
After that, you want to start the application like so (explanation of params below):
istaria.exe weblaunch_standard username password worldname biotenum port auth_server_IP public_key playername DefaultShard
username - This is your e-mail address
password - This is your password
worldname - This is your shard - expressed in lower case
biotenum* - This is your char's biote ID
port - Always leave this 0
auth_server_IP* - In form "ip: port" without quotes or spaces - you can see this in the launch URL
public_key* - This is that long as string of hex
playername - This is your character's first name
DefaultShard - This is always "DefaultShard" - don't change it
* You should be able to get this information from the second login screen from the Character management website: http://play.istaria.com where you used to be able to actually launch the game after logging in to the website. Simply right-click on the page's background and select "View Source" once you are at the "Start Istaria - Character: <toon name>" web page.
An Example Batch File:
C:
cd "C:\Program Files\Artifact Entertainment\Horizons"
istaria.exe weblaunch_standard myaccount@yahoo.com hehyeahright chaos 1234560 0 198.65.124.150:47101 0602000000A40000525341318001000001000100D30E97AC5D 826759CF495CD11A1127BF73B91EC860E656001340F3D20D19 96C1BCDDBDE8778BE494992FAUXCSAMPLEC4 ToonName DefaultShard
Note: the line used to launch the game, starting with istaria.exe, is actually all on one line. It is very long and will wrap around.
All you need to do is open up notepad and save the file as a .bat (Windows Batch File) and place it somewhere on your machine. You can run batch files by just double-clicking on them. I keep mine on my desktop due to easy access. Do not put it in the Horizon's install directory since it will get deleted during a full scan.