The MessageBox function just calls a Windows API function of the same name.

Your best bet is to create a Response type window that looks like a MessageBox window.

Create a structure object with all the options, title text, and message text. Then use OpenWithParm passing the structure to the window. Something like:

s_msgbox lstr_msgbox

lstr_msgbox.title = "Error"

lstr_msgbox.text = "Something went wrong"

OpenWithParm(w_messagebox, lstr_msgbox)

If you want to get fancy you could also create a function object with arguments similar to MessageBox that would populate the structure and open the window.

The biggest issue you'll have is resizing the window to match the message text.