you are doing it the wrong way.
The array should have tsharedref inside of it.
TArray<TSharedRef<STextBox>> TextBoxes
now you can actually do:
TSharedRef<STextBox> mytextbox = SNew(…);
TextBoxes.Add(mytextbox);
TSharedRef<STextBox> mySecondTextbox = SNew(…);
TextBoxes.Add(mySecondTextbox);
Then you can access it through TextBoxes[0], and TextBoxes[1] respectively.
You should actually read a tutorial on C++ (since I saw your other post too, and you are a bit confused about arrays)