How to make lot/Sr No field in Sales Order entry mandatory in World A7.3 ?

We have a specific process wherein invoice is raised immediately after sales order.The other steps like print pick slip,ship confirm,delivery notes are bypassed.We are using mandatory manual Lot/sr no assignment.Need to know how to make lot/srno field mandatory on SO detail line.I know there can be two options-1>Modification2>Add ship confirm in process .I am looking at other work around .Can anybody pls help?

I am attching screen shot for better clarity.

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We have a specific process wherein invoice is raised immediately after sales order.The other steps like print pick slip,ship confirm,delivery notes are bypassed.We are using mandatory manual Lot/sr no assignment.Need to know how to make lot/srno field mandatory on SO detail line.I know there can be two options-1>Modification2>Add ship confirm in process .I am looking at other work around .Can anybody pls help?

I am attching screen shot for better clarity.

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