The field that is added in the simulation for the user to type av will be a Text Entry Box. If you select the TEB object and look in Properties, there is a button for More Options. Set the Maximum Length to 2 characters and then you will see the Auto Submit checkbox become available. Select that option. If the correct answer the the TEB is set to av, then this setup will mean that as soon as the learner types the correct characters into the TEB its Success action will fire. You will need to set this Success action to jump to the next slide where you can show the drop down menu. A Click Box over the top of the correct menu item should be set to have a double click action.
If you set everything up correctly it should look fairly seamless to the user. However, be aware that you cannot always perfectly replicate the behaviour of actual software applications with a Captivate simulation. Sometimes you just have to let the learner know that the actual software will behave somewhat differently. They usually understand this.