Add dynamic data in Select dropdown using workflow designer

This may help:
https://developer.collibra.com/workflows/workflow-documentation/Content/Workflows/WorkflowDesigner/Forms/to_form-ex-3.htm

I have a similar problem. Let me know if you got it

The script task would need to create a list of map of the allowed values

For example, to gather the allowed Values of the OOTB Priority Attribute Type:

execution.setVariable('allowedValuesPriority', attributeTypeApi.getAttributeTypeByName('Priority').getAllowedValues().collect{[text: it, value: it]})

The form configuration would look like this:

Hope this helps