Pop-Up Form With User Input - Inventory Control

I am working on an Inventory control worksheet where i have all parts used in a certain assembly on the left (A) followed by Qty per assembly (B) and then current baseline Inventory (C). In columns D-max i will have at the top a pull down menu to let the user decide if they are subtracting from inventory (Purchase Order) or adding to inventory (Fulfillment). Under both circumstances, depending on which is selected I would like a different form to pop up which allows the user to enter values to control the chart.

If Purchase order is selected then a pop up will ask the order number, date, and quantity. This will then fill in three specific cells in the chart which control an equation to subtract the number of parts based on the order quantity.

If Fulfillment is selected I would like a pop up or the entire parts list to appear with a field to enter the number of parts being stocked. The program will then add these parts to the previous inventory entry.

In the example attached you can see that right now you have to manually enter the number of units ordered, then an If statement takes over to calculate the new inventory level (If statement used to keep things neat and possibly to incorporate fulfillment at a later time). At this time there is no way for me to enter stocking transactions.