Thanks for the clarification. In the previous reply I assumed that EquipmentInventory.Warehouse was a reference attribute of type Warehouse and that all warehouses would be represented in the system by instances of object Warehouse. Therefore the query would let the user pick a warehouse from a list of available instances and assign it to selected inventory items.
I now understand that EquipmentInventory.Warehouse is in fact a textual attribute that you want to set to the same user-entered value for multiple instances. To do it you can define a non-persistent object (say called IssueWarehouse) with a text attribute (WarehouseNumber) and modify the process as follows:
ENTER NEW IssueWarehouse
EquipmentInventory.Warehouse = IssueWarehouse.WarehouseNumber
The first rule will allow the user to enter the warehouse number, the second rule will assign it to all selected items.