How do I set a property in #systemd?

I have:

root@mx1:~# systemctl show postfwd | grep ^CanReload
CanReload=yes

I want to change that to no.
The fine manual says:
```
set-property UNIT PROPERTY=VALUE...
Set the specified unit properties at runtime where this is
supported. This allows changing configuration parameter properties
such as resource control settings at runtime. Not all properties
may be changed at runtime, but many resource control settings
(primarily those in systemd.resource-control(5)) may. The changes
are applied immediately, and stored on disk for future boots,
unless --runtime is passed, in which case the settings only apply
until the next reboot. The syntax of the property assignment
follows closely the syntax of assignments in unit files.

       Example: systemctl set-property foobar.service CPUWeight=200

But when I follow the example I get:

root@mx1:~# systemctl set-property postfwd.service CanReload=no
Unknown assignment: CanReload=no
```