Kindly note that the keyboard appears by pressing a combination of the "Steam" + "X" buttons as intended. Please note that the textfield should be selected (not just hovered above) by pressing the "A" button before using the keyboard combination.

If it does not work for you, please try the following:
Enable the Steam Deck's on-screen keyboard before clicking on the text field, then click on the text field, close the Steam keyboard after the field for text pops up, and enter your text using the default on-screen keyboard.