From my Scenario Editing Guide:
The placement of the centers of trade at the start of the scenario are defined next:
#Venice
cot = {location = 370merchant = { TAG = VEN level = 5 } merchant = { TAG = MEC level = 1} }
This defines the CoT in province 370, Veneto. The province number is given with the location statement. The number of merchants various countries have in the CoT at the start of the scenario is defined with the merchant statement, where TAG defines the country and level the number of merchants (max. 6).
Note that you can not define which provinces belong to a certain CoT. This is calculated based on the owners trade level and domestic policies.
To add a CoT to a scenario just open the scenario definition (the *.inc file), find the 'cot =' definitions and add it there...