Please use the api key that I've shared out of band. From here on out the api key will be referenced at {key}
To update a contact's last purchase date make a POST resquest to https://api.hubapi.com/contacts/v1/contact/email/{contact_email}/profile?hapikey={key} where {contact_email} is the email of the contact (customer) whose last purchase date we'd like to update
The Content-Type header should be set to 'application/json'