Two methods
One is to add it through online banking. Cards added in this way are called "online banking registered cards". Add the method "My Account > Account Management > Add Registration Card and Account". Just enter the card number and withdrawal password (6 digits) you want to add. But only U-shield or password card users can add it.
The second is to add it through ICBC branches without logging out of online banking (I often go to ICBC branches to do online banking services, and I feel that their staff are generally unfamiliar with online banking services). Cards added in this way are called "counter registration cards". When adding, you need to bring the card that has been used for online banking, the card to be added, and your ID card.