To change the reference field in the pre post report follow these steps:
1. Click Config, General Ledger
2. Define the Reference Field Name to Last Name or click Reset to System Defaults
3. Click Admin, Post to General Ledger
4. On the General tab, mark the Use journal reference from Configuration option

NOTE: This option is only available for Detail and Summary (Cash/AR only) options under "Post the transactions in"

NOTE: The total number of characters that the reference field can carry is 50.