iPad keyboard

You need a hardware keyboard, and you need to use the ⌘ or Cmd, known as the command key, instead of ctrl, or the control key. Most keyboard shortcuts on Apple devices use Command instead of Control. Eg; for copying and pasting.

On the onscreen keyboard, you cannot.

On a physical bluetooth keyboard, use the “cmd” key. Cmd+F will open a search field in Safari and other supported apps for example.