In order to create a Mule account shortcut, you will need your email and password for said account.
Below are 2 ways to make one:

Generate the string automatically


Enter your account email and password in the boxes below.
You can either do this in plain text or use an external base64 encoder.
(NOTE: THIS IS NOT ENCRYPTION, AND DOES NOT MAKE THE DATA SECURE)
Final string will appear here
OR

Create the string manually

1.Find your RotMG Exalt.exe directory. (Typically located in C:\Users\[username]\Documents\RealmOfTheMadGod\Production\RotMG Exalt.exe)
2.Make a shortcut to this file.
3.Open the shortcut properties and go to the "Shortcut" tab.
4.Find the field called "Target:", and add a space at the end.
5.Copy and paste this string after the space: "data:{platform:Deca,password:base64password,guid:base64email,env:4}"
6.Now, you need your base64 email and base64 password. You can use sites like this one to encode them into base64.
7.Go back to your shortcut and replace base64password and base64email with the values you got off the site.
8.Click apply and Launch the client from the shortcut. Note: this only works for DECA accounts, not steam. However, you can still open steam clients first and then open mules with the shortcut.

Credit: https://www.reddit.com/r/RotMG/comments/iy5cfy/guide_opening_multiple_rotmg_exalt_clients_at_once/, https://imgur.com/a/1afO1X3