Textbox shows asterisks as user types to mask a password or other sensitive information. You must add the stylebit _ES_PASSWORD and remove the stylebit for _ES_MULTILINE.

nomainwin

stylebits #main.txt, _ES_PASSWORD, _ES_MULTILINE, 0, 0
textbox #main.txt, 10, 10, 100, 30
open "Password Box" for window as #main
#main "trapclose [quit]"
wait

[quit] close #main : end