To be clear, in MSDOS, running on a monochrome monitor, AGI completely ignores the input prompt (i.e. it doesn't matter what you set string s0 to). The input box is hard coded to include the phrase "ENTER COMMAND" centered at the top of the box, with the single input line that displays only the input text and the cursor just below. No input prompt is included, but you can change the cursor character (using set.cursor.char).
The buffer for the input text is in the same location as for any other video mode, but it is just as inaccessible. So you can't modify it through code, in HGC mode, or any other mode*.
@Kawa, btw, in your image the input box has the caption "Enter input" not "ENTER COMMAND". I assume that screen shot is from SCUMMVM then? Because in all the MSDOS versions I have, the caption is "ENTER COMMAND".
I don't know a lot about SCUMMVM, but my experience has been that the modern interpreters don't accurately capture all the nuances of the original DOS interpreter, because errors and edge cases tend to be ignored in modern systems (they don't let you overrun memory, for example). So there are things you can do in MSDOS (or using DOSBox) that modern interpreters just can't handle.
*I have a 'secret project' that I'm just about ready to release that will demonstrate some of the cool things you can do with the original DOS interpreter by taking advantage of some of their poor coding practices in AGI. (Spoiler: the functionality that klownstein is looking for is easily doable if you know the secret!)