How to set javaagent in intellij idea VM Options?

You should put the file path in double quotes :

-javaagent:"C:\Program Files (x86)\StackifyPrefix\java\lib\stackify-java-sprn.jar"

Double quotes method explained by bowmore didn't work for me, had to go the slash way:

-javaagent:C:/Program Files (x86)/StackifyPrefix/java/lib/stackify-java-sprn.jar