The best way to achieve this will probably depend on the platform your existing site runs on. Ideally, the best option is probably to create a Netsuite restlet which the site posts to. You can then just send JSON lead data to the restlet securely - but you'll need server side scripting on your site to handle token based authentication (relatively easy with PHP). If you lack server side support, you can use a publicly exposed Suitelet script instead - but you run the risk of spammers locating the endpoint and posting spam directly to it.