How to use expression for field calculation in Access using VBA? -

i change expression of calculated field in access using vba. have managed create field values being determined expression. have used along lines:

sub test()      dim db database     dim fld field     dim tbl tabledef      set db=currentdb     set tbl = db.tabledefs("tblmytable")     set fld = tbl.createfield("identifier", dbtext)     fld.properties("expression") = "abc: " & "123"  end sub 

what have change expression existing field "identifier"?

thanks lot.