А правильно написано? Не должно быть “If you desire so.”?

Гугл говорит - правильно. )
Uly, Grumbler, можете объяснить почему? Какое тут правило? )

Thanks, Uly! )
My inner English learner felt like it was wrong. ))

Fixed expression = idiom? )

I don’t understand what’s the rule is about. It’s obvious where to put “if you so wish” in a sentence and what it would be called in linguistics. Did I miss something?

No, I’m talking about my sense of the language. I wouldn’t put this phrase in other way in these sentences. I took what you wrote as an explanation what this phrase is called in linguistics depending on its place in a sentence. Am I wrong?

Ok! That’s what I was talking about. ) I wouldn’t call that a rule.

Hey, Uly! Why did you delete your comments? I was checking the dictionary to say that I’m grateful for your wish to help me. )