You cannot use the diplomacy screen to encourage trade with a country the same turn you offer the consulate. Once you have a consulate a helpful trade policy can be set.
On the diplomacy screen, click on the turquoise trade policies oval at the top of the screen. This takes you to the trade policies panel.
Select the percentage of trade subsidy you wish to grant. The larger the percentage, the more the other country will appreciate your Empire. Of course this reduces your profits and increases the cost of commodities you purchase.
Click on the country on the map. A gold circle with the percentage amount appears on the screen. Trade subsidies take effect immediately.
Power tip: hold the control key down while clicking on the country. The computer will automatically select the correct subsidy to make you the selected country’s most favored trade partner.
