If Condition

Hi!

Need help on this please:

I have the following fields:

1. Property_address1
2. Auction_Venue (dropdown) with following choices: On-Line, On-Site, Live But Off-Site
3. Auction_address1

This is what I want to achieve:

If Auction_Venue is On-Line, Auction_address1 = "Online Only"
But if Auction_Venue is On-Site, Auction_address1 = Property_address1
And if Auction_Venue is Live But Off-site, user must enter a specific address.

Please help me with the script. THANK YOU so very much!

Regards,

Queenie