To add an instrument to your watchlist, you will simply need to click on the Plus sign which can be found next to the instrument.

Trade Markets

Trade Currencies

Trade Crypto

You may also add instruments to your watchlist directly from the Watchlist tab by clicking/tapping on 

Add New Market.

You will simply need a select a Market Category, select your instrument and click/tap on the Plus sign.

On the other hand, if you would like to remove an Instrument from your Watchlist, you would need to access the Watchlist and click on the Minus sign.