In short, you simply need to add a hidden field by the name of
Organic Script Campaigns
If you track the campaign using the organic script, the script will automatically inject the "event_id" in the td_eid field.
Redirect URL campaigns
If you track the campaign using the campaign redirect url, TrackingDesk will automatically append a parameter &td_eid=Event ID to the landing page url.
How to setup a hidden field in your landing page:
Your form provider should have the ability to automatically capture the parameter from the URL and inject it in the td_eid field.
Consult with your form or landing page provider technical documentation regarding the setup of hidden fields.