In Mendix 10.18 there is no off the shelf solution. Since Mendix 11.2, Workflow Groups was introduced https://docs.mendix.com/refguide/workflow-groups/ which can offer a solution for this use case
First, you save an association from your context to the selected approver.
Second, assign the selected approver user in the On created event microflow call of the of the Approve task.
The assignment might need some Java magic to assign automatically (and not by the user herself) if it is not allowed by default. In this case call the assign microflow from the Community Commons / executeMicroflowAsUser_1 as the approver as the user and the usertask as parameter.