You don't want to add in a variable and invert its valuable. You want to insert the "(!OS)" tag and make it being parsed by the other script. Therefore, you should add quotes around the (!OS), as it has to be interpreted as literal text to be added into the output string.
Although, there's a big change that this won't work at all. When you receive a message, Plus! will let all scripts do their stuff with it by calling their OnEvent_ChatWndSendMessage functions. However, you can
never make sure that your script gets executed
before the Computer Details script can find it.
The best thing you can do is make a quick text with the "/details" shortcut and make it send "My Operating System Is (!OS)". I understand that you're trying to get into scripting, but if you want to make it functional, you're better off with a quick text
or you can analyze the code of Computer Details and use it in yours. That way, you can let your script find out the operating system and add the result to the output message, so it won't depend on another script to make it function.
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