Sirkanth,
You are not supposed to enter the BP# there. On the left side, you see Find-> Select Business Partner
By->(External Number)- When you don't have the BP# in GTS and just the customer# in ECC.
In this case, you have mentioned 1208- Your external number in ECC and click on start. System will show you the BP# created in GTS. Just double click on that to see the details of the Business partner on the right side.
Regards
Dhilipan