I haven’t used the new text boxes much, but it looks like the main trick is that you need to get the focus on the textbox for it to accept keyboard inputs. Clicking inside the text box area might work, or pressing “tab” might work.
That said there are also workarounds if you’re doing everything in code anyway where you just manually set the ‘text’ property. Here’s an example using TextStim, but it could be adapted to a TextBox as well: Display typed responses - #2 by jonathan.kominsky