How can I change "_Cutoff" default value in shader?

It's not possible without doing it with code or manually editing the shader at the moment

Ok, I see.

Then how can I edit code manually? When I open shader in Shader Forge after I edit code manually, my code is disappeared because Shader Forge overwrite it.

So is there any keyword to prevent this?

There's no way to prevent SF from overwriting the code at the moment I'm afraid