Ok i just found the answer.

The 'atoms' (number boxes) cannot receive directly a message. I used the list2symbol object to convert it :

[receive mayVar2]
|
|
[list2symbol]
|
|
[hello \

and it works.

hello all,

thanks portabello for your example. This is what i need and it works fine with the windows cmd line.
But i cannot manage to pass a string instead a number.
When i pass :

pd -open "mypatch.pd" -send "; myVar1 45; mayVar2 hello"

i get the error :

error: gatom: no method for 'hello'

i tried alternatives such as :

pd -open "mypatch.pd" -send "; myVar1 45; mayVar2 'hello'"
pd -open "mypatch.pd" -send "; myVar1 45; mayVar2 \"hello\""
pd -open "mypatch.pd" -send "; myVar1 45; mayVar2 message hello"

but nothing worked.
Do you have any idea ?