One way to do this in your form is to use the
tag and set the method to "post". If you're not going to save your data, set the "nosave" attribute to "true".
The following example was contributed by Doug Landers and submits a test for to the Saleforce debug service.
<form>
<controls>
<hidden ref="updatemode">1</hidden>
<hidden ref="oid">salesforceIdHere</hidden>
<hidden ref="retURL">returnUrlHere</hidden>
<hidden ref="debug">1</hidden>
<hidden ref="debugEmail">debugEmailHere</hidden>
<input ref="wer" required="true" errormessage="This field is required."><label>This is a test...</label></input>
<addbutton text="Send" />
<redirect target="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="post" nosave="true" />
</controls>
</form>