How to read labelManagerCustomField options via API?


I am using the Atlassian.Sdk .NET Nuget to interact with our Jira instance. It’s working for the most part, but I can’t happen to read the allowedValues for a custom field created with the labelManager. As a workaround I read all tickets that have the field and created a list our of all values that appeared. But if a value has never appeared, then I can’t see it…

Any help would be very much appreciated! Reading it via Rest API would be totally fine, if its not possible using the Nuget.