The documents and issues didn't return relevant information directly about using Enums for setting a dynamic drop-down list in a Prefect deployment form.
However, interactive workflows in Prefect could be a helpful feature to explore:
Interactive Workflows.
For more detailed guidance, you can check out the Prefect documentation related to deployment:
Prefect Deployment.
If this does not answer your question, you might want to ask on the Prefect community Slack or forum for further assistance.