On ObjectKeystroke is a trigger you can use to perform a script with this script:
If [ Code ( get ( TriggerKeystroke) ) = 13 // 13 is code for "enter" key on Qerty part of your Windows keyboard]
Insert Calculated Result [¶ & "•"]
Exit Script [False]
If you want to detect the other "enter" key, the one that's part of the numeric key pad, test for code = 10. Exit Script [False] Cancels out the return character that will otherwise be inserted into the field after the script executes.