Hey NNF,
If you use the Standard Web-To-Lead Form you will not be able to add anything into the contacts table. You have to create a Custom Web Form and you will be able to add tags into the contacts table. Be sure to use the examples from the standard Web-To-Lead Forms to assist you with creating the Custom Web Form if you need assistance.
-- Loi
Loi Nguyen
Community Support Specialist
Method:CRM
l.nguyen@method.me
Toll Free: 1.888.925.6238
Local and overseas: 416.847.0400