Nintex PPK default value from JS

Hello,

I've got a People Picker control in the Nintex form (Share Point 2010).

I want to set the default value dynamically, with a value from a JS file which is connected to the form.

In the JS file, I have the user-Id of the default People Picker value, in a variable:

I gave the control CSS Class:

and tried:

But it didn't work.

Any ideas? please