Tie
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.
First method
Select first note:

+ creates a tie:

(+ key or the tie button located on the top to the right of the notes)
Second method
To create ties during note entry, press + after the first note of the tie.
Tied chords
To add ties between two chords, select the stem of the first chord, or Shift⇑ + click on the first chord and press +. note entry mode must not be enabled.
X flips the direction of a selected tie.