You need to look within the .sh to see where the packages are run to find the lines and the parameters it needs for it to run the specific forger version.
The console will complain that the .jar is missing then happily ignore its warning and start the instance
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Here is an an example of what works with my multicraft instance for forge 1.18.2-40.1.51
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[config]
name = Custom - Forge 1.18.2-40.1.51
category = Forge
[encoding]
#encode = system
#decode = system
#fileEncoding = latin-1
[start]
command = /usr/lib/jvm/java-17-openjdk-amd64/bin/java -Xmx{MAX_MEMORY}M -Xms{START_MEMORY}M @user_jvm_args.txt @libraries/net/minecraftforge/forge/1.18.2-40.1.51/unix_args.txt "$@"