Hey FujiMats,
If you are using Method:Classic then there is no way to do this currently with the web-to-lead form. However the Web-To-Lead form in Method:New does handle this, you can read more about it here.
Michael Melo
Product Manager
Method Integration Inc.
Website: http://www.method.me
LinkedIn: http://www.linkedin.com/in/MichaelMelo