In order to create leads on Hubspot, you will need to use the Contact API. You can either use /crm/v3/objects/contacts endpoint to create single contact or /crm/v3/objects/contacts/batch/create endpoint to create batch contacts.
Here development resource for contacts API, you can take a look for further information.
You can create a private app on your HubSpot portal and use its access token. Even you can perform the Hubspot OAuth process to retrieve the access token and refresh the token then, use them to work with the APIs.