Automation: How to assign an issue to the user from a custom field

[Chris Dunford]

When a rule is executed, we need to assign the issue to the user whose name is in a custom field.

Is there a way to do this? We don't see the custom field in any of the assignment options, including 'Another field value', which shows only standard fields like 'Issue creator' and 'Issue reporter'.

The custom field is defined as a user field, not just text.