Click on the coin icon at the top of the screen to reach the trade policies panel.  

A trade subsidy provides a percent change to the world price on all commodities traded with the selected country. If you are selling to that country, the percent change will reduce the price. If you are buying, the price you pay will increase. 

Using subsidies you can become the favored trading nation of a valuable Neutral State or Tribe even if some other Great Power has more friendly relations with them. This guarantees your traders the right to purchase the exports of the Neutral State or Tribe first.  Each trade improves the relations between the two countries.

Select the amount of the subsidy you wish to provide by clicking on one of the coins. Then click on the nation you wish to subsidize. You see a small icon confirming the subsidy. To cancel the subsidy, click on this icon.  

To provide a subsidy that will make your Great Power the favored trading nation of the recipient, select the 5% subsidy and hold down the control key when you click on the receiving country. The amount of the subsidy will be established at the minimum level to put your Empire in first place.

Boycotts prevent all trade between the two countries. Colony boycotts forbid your New World colonies from trading with the recipient, but do not prohibit your home country from doing so.

Your Empire must first establish trade consulates with Tribes and Neutral States (using the overtures panel) before you can set trade polices with those nations using this panel.