To see all custom fields that exist in the workspace, use workspaces/:id/custom_fields endpoint.
You can configure custom fields for your projects and tasks. The custom fields for a given item are returned as a hash, and are settable in the same way.
Suppose you have a custom field "Urgency" with values of "High" and "Low". Then if you get a task whose Urgency is High, you will see:
and you can change the Urgency to Low with a PUT that includes:
as one of the attributes.
To clear a custom field value, just use empty quotes for the value, for example:
% curl -X PUT https://app.liquidplanner.com/api/workspaces/:id/tasks/:task_id -d