http://purl.org/tcl/home/man/tcl8.4/TkCmd/chooseDirectory.htm
also see tk_chooseDirectory . [What is the difference between this and tk_choose Directory?] Are there issues when choosing new vs existing directories?
Does this routine support the case where one wants to select a subdirectory of a not-yet existing directory?
Category Dialog - Tk syntax help - Arts and Crafts of Tcl-Tk Programming