There seems to be a annoying bug in the UX in Security Center when adding indicators.
Go to: Settings > Indicators > URLs/Domains and choose Add item. Enter any domain and click Next. On the Action page, choose Alert and block. Enter any text in "Alert title" and then select this text by pointing the mouse from the end of the text to the beginning of the input text without letting the mouse button go, then move the cursor out of the fly-out and release the mouse button. This makes the UI disappear!
Then the natural thing to do is to go back and try again so click Add item and then enter any URL and go Next. Then this UI page is frozen and you cannot select Alert and block! You must reload the entire page by hitting refresh in the browser.