Digital Trigger B (TRIG1) does not need to be used when you only need one trigger. Digital Trigger A will function as a start OR stop trigger, but if you need a start AND stop trigger, Digital A and B need to be used.
In your case, you need to acquire both pretrigger and posttrigger scans. This is a stop trigger by definition, and since you didn't mention that a triggered hardware start needed to be implemented, you only need to specify the Trigger Type as "Digital A." You can then specify the Total Number of Scans to Acquire and the number of Pretrigger Scans. The number of posttrigger scans acquired will be the difference between those two values.
Hope this helps.