gistfile1.txt

function webToLead(firstName, lastName, email) {
    var form = document.createElement("form");
    form.method = "POST";
    form.action = "https://test.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8";   

    // Your org ID
    var elementOID = document.createElement("input");  
    elementOID.name="oid";
    elementOID.value='';
    elementOID.setAttribute("type", "hidden");
    form.appendChild(elementOID);  

    // Whatever params you want;
    // generate a form from Customize | Leads | Web-to-Lead to figure out more
    var elementFirstName = document.createElement("input"); 
    elementFirstName.name="first_name";
    elementFirstName.value=firstName;
    elementFirstName.setAttribute("type", "hidden");
    form.appendChild(elementFirstName);  

    var elementLastName = document.createElement("input");  
    elementLastName.name="last_name";
    elementLastName.value=lastName;
    elementLastName.setAttribute("type", "hidden");
    form.appendChild(elementLastName);

    var elementEmail = document.createElement("input"); 
    elementEmail.name="email@email.com";
    elementEmail.value=email;
    elementEmail.setAttribute("type", "hidden");
    form.appendChild(elementEmail);  

    document.body.appendChild(form);

    form.submit();
}