Historically, to create Lead API forms, you had to access the Lead Director (HubSpot Lead API) from Settings, under Integrations:
The Leads API is deprecated and getting replaced by the Contacts API so that you can take advantage of the more robust features and integrations associated with HubSpot 3. With this change, the forms in your HubSpot Lead API screen will show in the new Forms app where you can access all your old and new forms.
You will find your old API forms in the new Forms interface. They will be fully functional and are going to remain active until you manually disable them.
When you click through on one of your old API forms from within the new Forms app, you will see the form's name and whatever email notification addresses you had added to it. Don't be worried that there are no fields in the form, the information is still going through into your Contacts database.
In order for your old API forms to feed contacts into your Lead Nurturing campaigns, you'll need to migrate from the classic Lead Nurturing tool to Workflows. If you have done this already, you are good to go! If not, please make this migration a priority: it's a one-click process, after which your forms will contnue working with the new Workflows.