You will have to create code for the OnCurrent event or BeforeInsert event. You can test to see if the UniqueID has a value. If "no", then use code to create / find the UniqueID.
Another approach, if you use a "smart" UniqueID, is to build the the entry after you have enough information. For example, after the customer has been selected, and the date and time is set, you can use the value of these three fields to create the UniqueID.
CustomerCode + yyyymmdd + hhmmss
How to create the UniqueID will be up to you since the post is shy on details.
Richard