Specify which additional parameters you'd like to pass to packer, these arguments are in form: -var varName=${ENVVAR} or -var varName=value
Example: -var aws_access_key=MYACCESSKEY -var aws_secret_key=MYSECRETKEY

If using a variable for an argument, make sure this variable can be accessed through Environment Variable of Jenkins Build.

For more details consult Packer documentation.