From your project access Project Administration on the bottom left menu

From admin locate Workflows and select the Workflow you want to change

Here we used Add Transition in the top menu bar called escalate and connected Open and Done

We selected the connector between Open and Escalate and then selected Post Functions in the transition edit box

From Post Functions option select add a Post Function

Select Escalate issue post function

Select your roster and select add

Once saved you will need to save your new workflow and backup the original if required.

You will see there is now an Escalate option in your issue workflow

Select escalate and the roster will be invoked and the issue assigned accordingly