Validate and allow SO numbers in frontend

frontend/src/routes/guest/register/steps/register.tsx

@@ -590,7 +590,7 @@ const GuestRegisterStep = forwardRef(
                   // It is not required that the Norwegian national ID number be filled in, the guest may not have
                   // one, so allow empty values for the validation to pass. Note that both "fødselsnummer" and
                   // D-number are allowed as input, and in some cases SO-number
-                  validate: (value) => isValidFnr(value, true),
+                  validate: (value) => isValidFnr(value, true, true),
                 }}
                 render={({ field }) => (
                   <TextField