How to add java args *after* the "java -jar minecraft.jar"?

Isn’t what you’re asking for the effects of the “Additional Jar Arguments field?”

So if you fill in:

Additional Java arguments: “-passedToJava”
Additional Jar arguments: “nogui”

then the resulting invocation should look like:

$ ./mineos_console.js -s asdf get_start_args
[asdf] Successfully executed "get_start_args"
[ '-dmS',
  'mc-asdf',
  '/usr/bin/java',
  '-server',
  '-Xmx512M',
  '-Xms512M',
  '-passedToJava',
  '-jar',
  'minecraft_server.1.15.1.jar',
  'nogui' ]

This is how I normally add nogui to the vanilla jarfile, but note it’s also nogui not -nogui.

$ cat server.config 
[java]
java_xmx=512
java_tweaks=-passedToJava
jar_args=nogui
jarfile=minecraft_server.1.15.1.jar
...

I’m just speculating, but I believe the leading hyphen - might even force arguments to be parsed by java rather than as an argument to -jar. If this is the case, it would explain why vanilla minecraft pushes simply nogui.