This field is fully replaced on every request, always send the complete set of data you want stored. If it is omitted from the request body entirely, existing data is left untouched.
The serialized data is capped at 10 KB and can hold up to 30 dynamic fields.
optionalusername
username?: string
The user's username in your system (max 1024 characters)
https://docs.discord.com/developers/resources/application-identity-profile#update-application-identity-profile