A tie connects two consecutive notes of the same pitch and should not be mistaken for slurs that connect multiple notes for other reasons.
The note-properties menu let you start and end ties at will. You can start a tie with the shortcut <9> (the key usually has the open bracket as well written on it) and ends with <0>. If both notes are available already either insertion of the ending or the beginning is sufficient.
It is possible (for reasons of compatibility with earlier versions) to use the tie-mechnism to enter slurs between notes of different pitch.