How to send UnityWebRequest with Date Header under webgl?

I know that setting Date is not allowed by UnityWebRequest because it is managed automatically.
But even if I don't set the Date manually, UnityWebRequest doesn't add it automatically.

Therefore I want to switch to other tools, such as .NET networking classes.
But I noticed that this doesn't work in WebGL because IP Sockets cannot be manipulated directly.

The only way I can think of at present is to customize the request through JS code in the form of jslib, but this is too troublesome.

Is there any other feasible way?

I apologize for my ignorance, I wrongly blamed untiy, which is Fetch's forbidden header, and has nothing to do with Unity. But what is strange that Fetch didn't automatically add Date?