Configuring Custom Field Mapping
Suppose an Issue has a custom field with the reference name TEST, which you added in the project settings. To synchronize its value with the matching field in the external system, use the following mapping settings.
Redmine
You need the identifier of the custom field in Redmine. Open an issue card by its link and append .json, for example, https://redmine/issues/1452.json
In the "custom_fields" section, find the id field. Use it in a substitution of the form:
"TEST": "issue.custom_fields:(id=42).value"