Lenght string

[Montana Bag]

I need to get length of string in attribute.
I found something here:
https://networkradius.com/doc/3.0.10/unlang/xlat_length.html

I've tried:
Tmp-Integer-0 := %{#control:Tmp-String-0}
Tmp-Integer-0 := %{#:control:Tmp-String-0}
...

but nothing happened:
/etc/raddb//policy.d/dhcp_opt82[299]: Unknown or invalid value
"%{#control:Tmp-String-0}" for attribute Tmp-Integer-0
/etc/raddb//policy.d/dhcp_opt82[299]: Unknown or invalid value
"%{#:control:Tmp-String-0}" for attribute Tmp-Integer-0

How to do it right?