Dennis @ PerfectForms
You can do this using the Random function, a few helper fields, and a table (like Macgyver).
Create a table with 36 rows, one for each letter of the alphabet and 0 through 9.
Generate a random number between 0 and 36 and go to that row # of the table. To do this build a looping behavior that starts at the first row, then goes to next row, etc, and counts each loop until you have reached your random number.Take the value in that row of the table and add it to your result field.
Run this behavior enough times through until your GUID is the proper length.
You will also have to build behaviors that add your “-” characters at appropriate intervals.