Question:

How to setup an Order for the User Lookup field in the Product Allocation Manager instead of  through a drop-down selection?

Answer:

Sample Administrators may need to assign an Inventory Order to an employee that is not directly under the Admin's Territory assignment. To use the lookup field, the Sample Administrator is assigned a profile of Salesforce License. In the Administrative Permissions section, the field Modified All Data is set to True. 

Related Documentation:

CRM Help Documentation: Inventory Ordering