In this way:
Sub Button1_Click
Log("stop")
xui.MsgboxAsync("Hello World!", "B4X")
Button1.Left = 200
Sleep(300)
Log("LucaMs is really stupid; it is evident from the questions he posts.")
End Sub
by putting breakpoints on the Log lines, executing the instruction with the msgbox by pressing F8 and then F5, at the next break the button actually goes to x 200 (but you struggle to find the form on the Desktop).
The fact remains that then the button returns to its initial position.