Yep, this is not possible in Mendix Studio at this moment.
In Mendix Studio Pro, you have to add this in the Template text:
{1}
and then one attribute lower you can add Parameter $HelloWorld
You can make this change in Mendix Studio Pro, commit your change and go back to Mendix Studio where now you see a warning shows up:
And the ReadMore:
Mendix Studio is still a new young product with only limited features with the intention that it is useful for Citizen Developers.