Flutter: testing, how to simulate the insertion of a text by the user

I have this widget that I want to test:

StreamBuilder(
      key: Key("usernameField"),
      stream: _bloc.usernameField,
      builder: (context, snapshot) {
        return TextField(
            onChanged: (val) => _bloc.usernameAddition.add(val),
            decoration: InputDecoration(
                prefixIcon: Icon(Icons.person),
                labelText: 'Username',
                errorText: snapshot.error));
      },

but I don´t know how to simulate the insertion of text by the user. I tried to insert the values directly to _bloc, but the test fails (and also is not what I want to achieve, but was just a try):

 _bloc.usernameAddition.add("a");
    await tester.pump();
    _bloc.usernameAddition.add("");
    await tester.pump();
    expect(find.text("Insert username"), findsOneWidget);