DRI_PRIME is an environment variable. DRI_PRIME=1 gimp is a bash syntax for setting an environment variable for a specific command execution, Whisker probably doesn’t use shell commands, it’s expecting an executable. So it’s trying to look up a file named DRI_PRIME=1 in your PATH.

You’re going to need to figure out if whisker has a way to set env vars for a command (I don’t use it so not sure what it shows when you click the “Help” button). If not, you’ll need to create an executable script file which executes your command with the environment variable set like you’re attempting to do (DRI_PRIME=1 gimp), and then point whisker at your script.