Sure, I can help with that! You can create an Apex trigger on the Account object that will update the account number field with "test" plus a random number whenever a new Account record is inserted or an existing Account record is updated. Here's some sample code that should accomplish this:

```java
trigger UpdateAccountNumber on Account (before insert, before update) {
    for (Account acc : Trigger.new) {
        // Check if the account number field is blank
        if (String.isBlank(acc.AccountNumber)) {
            // Generate a random number between 1 and 99999
            Integer randomNumber = (Integer)Math.floor(Math.random() * 99999) + 1;
            // Update the account number field with "test" plus the random number
            acc.AccountNumber = 'test' + String.valueOf(randomNumber);
        }
    }
}
```

This trigger will run before new Account records are inserted and before existing Account records are updated. It will check if the AccountNumber field is blank and, if it is, generate a random number between 1 and 99999 and update the AccountNumber field with "test" plus the random number.

Please note that this trigger will update the AccountNumber field for all new or updated Account records, even if the AccountNumber field was already populated. If you only want to update the AccountNumber field for new Account records or for existing Account records where the AccountNumber field is blank, you can modify the trigger accordingly.
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