Ok, I figured it out. I was a bit confused but now I'm fine. I did, with the Execute SQL function->Calculated SQL text:
"INSERT INTO `users` ( `id` , `username` , `password` , `reply_to` , `reply_name` , `last_login` , `cookie` )
VALUES (
'', '"&WDev::Navn&"', 'password', '"&WDev::Epost&"', '', '', ''
);"
WDev being the name of the FileMaker database.
It was the '"&WDev:<field>&"' part I couldn't figure out right.
Thanks ya'll!