If you are using a search field that allows you to select from a list of options (Cities, for example) and you need to select more than one item:
On a Windows-based computer, hold down the CTRL key as you click on the items you want.
On a Macbook or iMac, hold down the Command key as you select the items.
On an iPad, simply tap on the items you want. You do not have to hold anything else while you tap.