source migration to custom field set on user account


I wrote a simple user migration of user data (email, status, Name) which is working. I also have a custom field (field_ipaddress) added to the account but I am having difficulty migrating to it. 

I tried adding the code to the main migration which makes use of an entity: user destination and a separate migration after the main migration which is using the destination of entity:field_config.

The module maintainer has gone MIA and so it was picked up by another dev and stored on Github here.

Any suggestions?

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