Next, highlight the column(s) that you've selected for your table's primary key. Then click the key icon in the taskbar to set the primary key. If you have a multivalued primary key, use the CTRL key to highlight multiple rows before clicking the key icon.
Once you've done this, the primary key column(s) will have a key symbol, as shown in the image above.
If you need assistance, you might want to read the article Selecting a Primary Key.