Nope. While you can modify attributes of the user object and the changes will be persisted, you can't modify the registrations of that user.
Can you use an include for the function body? For example: (where myLambda.ftl is your Lambda function in a folder named lambdas)
That should preserve your line returns if you include it that way. Hope that helps!
We do not support this directly in FusionAuth, but you could use a Lambda to set the aud claim to whatever you want.
The specification allows for a string value, or an array of string values, so you could create a Lambda with something like: jwt.aud = [ 'foo', 'bar' ];
More about Lambdas and JWTs here: https://fusionauth.io/docs/v1/tech/lambdas/jwt-populate
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