In the form where you choose the manager's name,
Place a textbox and set’s attribute to manager ID and make it non-editable,
There are two ways, from which you can do it,
When you choose Manager's name, call a microflow, retrieve the manager you selected, and change the manager ID.
on the change of the manager name, call a microflow and refresh the dataview entity,
Let me know if you face any issues,
Hope it helps!
Having ManagerID on the Employee entity is not recommended as you always can get it through association.
However, if you really want it, you can add an onchange event to the reference selector and use a microflow/nanoflow to set the Employee’s ManagerID of the selected Manager.