Hello A.,
I am extremely sorry if this is a very trivial question.
No pb. All question are welcome...
A possible solution, is to use empty label as:
@startuml
label " " as l0
rectangle "System" as s #gray
label " " as l1
l0 -> s : Input signal
s -> l1 : Output signal
@enduml
See doc here:
If that can help,
Regards.