vba - Formatting fields to add dashes in SSRS -


i pulling string of numbers database , wanting format them dashes between them. more specifically, want them go looking this:
1234567890
to this:
12345-67-890

i've tried change format through placeholder properties, it's not working. think might because string.

my code looks this:

format(fields!num.value, "#####-##-###") 

and #####-##-### returned when run it.

you can use expression apply formatting numbers. can either use expression directly in report or add calculated field dataset if referenced in multiple places.

=left(fields!num.value, 5) + "-" + mid(fields!num.value, 6, 2) + "-" + right(fields!num.value, 3)