Auto Populate Field

[DM - NPS]

I have 2 fields 1 called "Trail" and another called"EventID". What I want to
be able to do is have the user enter a trail name and if that trail name
(picked from a pick list) is the same as the previous record then the EventID
field automatically populates with the EventID from the previous record. If
the trail names are different then the EventID automatically populates with
the following: Format(Now(),"yyyymmddhhnnss") & '-' & 1000000000*Rnd(Now())

I am very new to code so if you can give me an example of the code or point
me in the direction of an example I would really appreciate it.

 

[KARL DEWEY]

The problem is how do you determine which record is considered the previous
one.
How are you sorting the records?
Post the SQL of your query and sample data.