The script seems to assume that %1 contains a source file. If I remember correctly, "Send To" sets %1, %2, etc. to the paths of the selected files. That's a custom behavior specific to "Send To" and Windows Explorer selected files.
I don't know how you configured Clavier+ to invoke the script, but unless you did something special, it's unlikely to preserve the custom "Send To" behavior. I recommend modifying your script to print the value of %1 and understand what's happening when Clavier+ invokes the script.
Anyway, this question seems unrelated to Clavier+, which behaves essentially the same way as the standard Windows "Run" dialog. So you want to find a solution that works with "Run" or by simulating keystrokes -- it's up to you to find a solution, since the question is about Windows Explorer selected files, not Clavier+ -- then configure Clavier+ to use this solution.