After clicking on the mentioned button you will be lead to the Delegation center of the wallet. There you will find all important information concerning delegation, the amount of ADA you will be able to delegate, a list of staking pools and an overview of the rewards you earn from staking.
To delegate your funds you will have to make a choice, to which staking pool you want to delegate to. Therefore you need to click on the button „Stake pools“ as highlighted in green.