Can't specify default value in enums for SDK

I had an enum with only one value, so it was treated as a constant. That meant customers didn't have to provide it and it was always set.
After adding new values to the enum I was hoping there was a way to add a default value to continue the same behavior (If not set, then it would take the first one).
I tried with "Default" and "x-ms-client-default" but didn't work.

I was hoping this:
public FactoryIdentity(string type = default(string)){ ... }

to be this:
public FactoryIdentity(string type = "DefaultValue"){ ... }

No source file, generator entry point, or test is named. Start by tracing how enum definitions and the Default or x-ms-client-default extensions are consumed during C# SDK generation; done means an omitted enum argument produces the requested default value and is covered by a relevant generation test.

Written by the indexing model from the issue text.