DSOstrider The numbers do not appear in the dialog box
This is what I see (ASIAIR v2.0, general release) on an iPad (12.9" M1 based iPad Pro, iPadOS 16.1.1):

When I tap on the Main Scope Focal Length text field, a flashing red vertical cursor at the end of the entry (see above screen shot). This is the usual iOS/iPadOS behavior.
At the same time, a numerical keyboard appears at the bottom part of the screen.
Notice the backspace icon (an open arrow with a "x" in it) at the right side of the virtual keyboard. First, tap that backspace button many times. (20 or 30 times, just for the heck of it.)
Then enter your focal length (the "3" in my example above is correct, since this is a 3.2mm lens of my all sky camera and ASIAIR truncates to the nearest integer).
You should see the numbers are they are typed (tapped) in, with the red flashing cursor remaining at the end of the numbers. If the field remains black, but the red cursor moves, check the Text Accessibility settings of your iPad (see end of this posting).
When you are done, be sure to tap on the "enter" icon at the bottom right of the virtual keyboard (the one that looks like an asymmetrical "V"). That should also exit the virtual keyboard.
Which of these steps failed for you?
Be sure to check also the Accessibility Settings in iOS. Especially the items in the Display & Text Size. If "Smart Invert" or "Classic Invert" are on, turn them off.
Chen