Password in asterisk format

Question

hi,
Very precisely am working in Expression Blend and there is no such property of textbox to show the text in asterisk form for quoting passwords and to incorporate this I used the following lines of code.

usertxtpass.attributes.add("value",passwordvalue);
usertxtpass.text = usertxtpass.attributes["value"].tostring();

but there is no such property like .attribute.add and .attribute["values"] so cannot use this too.. please suggest some solution other than this keeping Expression Blend in mind...?