Set 'Related To' field in sendEmail

Question

I am able to send email over the SOAP api using something similar to http://wiki.developerforce.com/page/Partner_Send_Email

However, I would like the emails to be associated with a particular Opportunity.

If the email is done through the UI this is possible and the email appears under the Opportunity's Activity History.

Is it possible to set this over the API?

Using targetObjectId gives Only Users, Contact or Lead allowed for targetObjectId

Answer 1

You can set the whatId property to the ID of the related Opportunity.

Answer 2

Normally I would say set the WhatID property, but I'm not seeing that as an option in the SOAP API developers guide.