Typically you will have the option to nominate a vessel on the trade dashboard before proceeding to the main terms.

Once vessels(s) have been nominated, it shows like this on the dashboard.

Then when you proceed to the main terms, all you need to do is paste in the details in the left pane under the vessel section, all fields inserted will be automatically populated to the termset you have chosen on the right.

However if a vessel cannot be found on the system, you can only go with TBN option which the vessel section on the left will be frozen and won't let you enter details.

And you can only insert details on the termset.

Click on the relevant term on the right termset pane, remove everything from the box and re-type in all the information needed