CIRD-IP Address

I am new to terraform, so any help is appreciated.
I want to write a variable which ask the user what IP/CIDR he wants to use for his Azure address space or/and address prefixes. So instead of the variable set it up for him like here:
resource “azurerm_subnet” “subnet” {
name = var.subnet_name
resource_group_name =
virtual_network_name =
address_prefixes = [“”]
I want the variable to ask which address he wants to use, so he can enter any CIDR he wants.
Thanks before hand,