Hi. According to some of the documentation it implies that I can use a custom fields' name to reference it. I can directly reference common fields as a $select parameter, but can only reference other fields using their custom-xxx tag, like this:

'$select' => 'First name, Last name, Email, custom-8097667,
custom-8097668, custom-8161502, custom-8097675, custom-8017535'

The custom field tag does not seem to appear anywhere in Membership fields area of the WA database though so the only way I can find it out is by trawling through json data manually.

Is there a better way to reference those fields, or to find out their custom-xxx reference (as I've mentioned, I've looked through the WA membership fields in the actual WA site and can see no obvious use of this custom-xxx referencing). The code snip below is part of the json returned on a full dump, and SystemCode is the only place that I can see that references the custom field number:
json data referencing a custom field:

[6] => Array
(
[FieldName] => Practice Address 1
[Value] => 40 Test Way
[SystemCode] => custom-8097667
)