JAVA validation required

Hello,

I have created a SO form in which I have 2 fields

SHIP-TO-PARTY & SOLD-TO-PARTY (having Dropdowns)

My requirment is If I enter any data to SHIP-TO-PARTY then the same data should reflect in SOLD-TO_PARTY

But the User can customize the data of SOLD-TO-PARTY If he requires.

Please send the java script for the above mentioned validation.

Thanks in advance,

Lina khanna