Key stroking fn + f8
Key code 100 is actually F8, If you want to press F8 key just send key code 100. 
But a different media control command key. Unfortunately, it seems the only way to run the pause command in media players via applescript is to send the proper key directly to the application(i.e. iTunes and Spotify). please see: 

https://gist.github.com/NoobsArePeople2/5121597

PS: If you want to check press Fn, you can run this script

delay (1.0)
tell application "System Events"
    key code 63 -- fn
    key code 63 -- fn
end tell


this press Fn key two times and trigger the dictation