The offer button sits next to the list of interpreters when looking at an appointment. Please note currently the offer button is only displayed after the appointment is initially created.
If you select - All Interpreters - and click offer, it sends offers to everyone on the list (qualified interpreters) that are available. Bookings, weekly availability, and vacations will stop the offer from going to the interpreter.
Select an interpreters name then click offer. The interpreter will receive an offer, it will not be blocked based on their availability. You'll notice the drop down list automatically changes their name back to - Any Interpreter - to keep that interpreter from being assigned if Save is clicked.
Navigate to Settings => Notifications => Email When Offered, from this page customize the email to meet your needs. There is an 'Auto Assign First Acceptance' switch that will automatically assign the first interpreter to accept the appointment offer.