On the Create stack > Specify Details page, you can either create a new trail and S3 bucket or use an existing trail.

First, create a Stack name or choose the default

If you plan to separately integrate multiple accounts, the Resource Name Prefix must be different for each account as S3 bucket names are globally unique.

Resource name prefix should be pre-populated with your account name. It doesn't need to change unless:

you are creating a stack for each account in which case it needs to be unique for each account as the S3 bucket namespace is global, or
you are creating multiple stacks in which case it needs to be unique to avoid resource collision

ExternalID is populated by Lacework, but you can change it if you follow a defined naming convention
Set Create a new trail? to Yes.
A Log file prefix can be added if you want a specific S3 bucket path for your logs.
Bucket name and Topic ARN should be left blank.
Click Next.

Integrating and using Lacework for AWS