A tie is a curved line between two notes of the same pitch. If you want a curved line that spans multiple pitches, see Slur.
Select first note:
+ creates a tie:
(+ or the tie button,, located on the top to the right of the notes in the note input toolbar)
To create ties during note input, press + after the first note of the tie.
To add ties between two chords, select the stem of the first chord, or Shift + click on the first chord and press +. Note input mode must not be enabled.
X flips the direction of a selected tie.