Instructions can be found at https://www.3ds.com/terms/software-keys/  to generate your Computer ID / Target ID. All valid Target IDs have the following format: 3 letters + “-“ + 16 hexadecimal characters [0-9][A-F].
For example: “JZR-423010145B64B1A4”.
Optionally, view this video, which illustrates the process:
> DSLS target ID tutorial