Under Pool click on Add. You will find the Liquidity interface. When you add liquidity you provide liquidity to the trading pair with a 50/50 ratio. This ratio is automatically calculated by the swap and you just have to enter a metric to have the other one filled up. For example, on the GLMR/EFT pair, I just entered the amount of GLMR I want to commit, and the amount of EFT needed to reach this 50/50 ratio have been filled automatically.