How to route Requisition Approval through Descriptive Flex field values?

Summary:

We have requirement to route approval though manual entry of Approver and it will be routed to particular user whose name entered in DFF.

We created DFF with LOV values showing user name from HR table to select Approval but when we are writing approval rule with DFF its not working as value from DFF is not triggering rule.

Let us know how we can write the rule with DFF value.