Testing on mobile has its complications. I don't think there's a way to just "summon" the keyboard without a text input object being focused. But the keyboard plugin doesn't pick up keystrokes from a text input, so that's not really going to help.
That's what I've discovered, at least. For my own purposes, I just use on-screen buttons and sliders, and map them to my debug commands. Put them on a separate layer that can be toggled with a button. If you really need a keyboard, then you can use a Bluetooth keyboard or a USB keyboard with an adaptor if necessary. It's messy, but it works!
Edit: I only just understood your question, sorry about the wrong answer! You're asking about Construct itself 😅