At the moment it’s not possible to search for a specific note when using the insert link button inside the “actions” tool bar item on top of the keyboard on iOS. If you have lots of notes, you won’t find anything by hand.
Unfortunately while we’d like to have added it as well, the fact that the popover is attached to the keyboard, doesn’t allow us to switch to another keyboard input view as it would dismiss the popover. So for technical reasons it’s not possible I’m afraid in the current setup.