How to initiate a specific keystroke using automator?
You can use the keystroke scripting command available through the System Events application.

Add the following block to your script below (and at the same indent level as) the activate:

tell application "System Events"
    keystroke tab using {command down}
end tell


This will simulate cmd+tab and have the effect of switching applications.

To simulate the "ALT" modifier key, use option in place of command in the above.