One of the sections within the Know Your Client area of contact records is the Personal Interests section. Note: unlike much of the Know Your Client information, Personal Interests are not accessible from a contact's Overview page. You would need to first click Know Your Client from your Left Menu Bar:
Then, Personal Interests are accessible by clicking "Misc Client Info" on the left-hand side of this section:
To add Personal Interests, just click the Add button in the top right corner of this section. You'll then see the below:
Personal Interests that have already been added for the contact will display on a blue background and have a checkmark. You can scroll through the other available personal interests or use the "Search Interests" box to find others to apply. You can also add new Personal Interests to your database to apply to your client using that Search Interests box. After making your selection(s), simply click add.
To later remove a Personal Interest from a contact, just come to this same area, click the corresponding Actions menu for that Interest and then click "Remove Interest":