How to pass custom user properties to amplitude in experiment.fetch()

We have a requirement where we pass city to amplitude in the fetch() API call when experiments are initialized.

We want the experiments to be created based on the city value that we passed, and not from what amplitude reads from the IP address.

We do not want to use the city based on IP address since it is not accurate compared to our DB

We are initilaizing using remote evaluation

So my question is

1. How to pass custom user properties like city, region to amplitude in fetch() call in Node SDK
2. How to create a 50-50 A/B setup for a particular region