The first step is to identify the workspace Object ID that the entity type exists in.
Open a browser and paste the following URL identify the Object ID of the workspace:
https://rally1.rallydev.com/slm/webservice/v2.0/workspace?query=&start=1&pagesize=2000&fetch=<OBJECT_ID>
Next, it is necessary to get the _ref value from the User Type Definition object. Use the following link, changing the <WorkspaceOID> to match yours. If you're using a different entity type such as Defect, replace User with that entity type.
https://rally1.rallydev.com/slm/webservice/v2.0/typedefinition?workspace=https://rally1.rallydev.com/slm/webservice/v2.0/workspace/<WorkspaceOID>&query=( Name = "User" )
The _ref link you are looking for will resemble the following:
https://rally1.rallydev.com/slm/webservice/v2.0/typedefinition/<OBJECT_OID>/Attributes
Copy that link and add a pagesize to the end so that it looke like the following:
https://rally1.rallydev.com/slm/webservice/v2.0/typedefinition/<OBJECT_OID>/Attributes?pagesize=2000
On this page, look for Department and get the _ref to the AttributeDefinition. That link will look like this:
https://rally1.rallydev.com/slm/webservice/v2.0/AttributeDefinition/<OBJECT_OID>
Now you are ready to use a REST API client to craft your request
Endpoint: https://rally1.rallydev.com/slm/webservice/v2.0/allowedattributevalue/create
Payload:
{
"AllowedAttributeValue": {
"AttributeDefinition": "https://rally1.rallydev.com/slm/webservice/v2.0/attributedefinition/<OBJECT_OID>",
"StringValue": "Whirling Dervishes",
"ValueIndex": "6"
}
}