Serial and Lot control

I am trying to understand how lot and serial control are related. When you do a Misc receipt and click on Lot/Serial button, the Lot number entry forms shows first. But I want to default the lot number based on a segment of the serial number. I have tried to use forms personlaization to default the lot number on the MTL_TRX_LINE form and it does but when you click on the Lot/Serial button, I get a message 'You cannot create records here'. It eventually opens the serial entry form. I appreciated any insight into how the form logic works for Misc Receipts.