Add a custom attribute in a pre-signup lambda function

I have a User Pool that has a custom attribute "custom:userId". When using the User Migration Lambda it is easy to edit the event.response.userAttributes to include the custom attribute. However, federated identities don't trigger the migration lambda, so I have set up a pre-signup lambda function. This does not have userAttributes in the response section of the event. I have tried adding "custom:userId" in the event.request.userAttributes but this does not reflect in the user. How do I add a custom field to the user in the pre-signup lambda?