You are sending an email to a coworker requesting that the coworker perform a task. where should you put your coworker's email address?

It really would depend on what E-Mail system you're using. 
If say, you're using G-Mail, the Recipient's address would be placed simply in the "To" bar right above the "Subject" bar. 

I’d say platform as a service (paas) or application platform as a service (apaas) or platform base service is a category of cloud computing services that provides a platform allowing customers to develop, run, and manage applications without the complexity of building and maintaining the infrastructure typically associated with developing and launching an app.