119: Playback - sustain pedal effect without using line

To get a pedal marking to playback but not display on the score you can do one of two things:

In Sibelius 2 (and later) you can insert a line from the Create > Lines dialog box and then hide it by right-clicking on it and selecting Hide or show > Hide (or press Ctrl + Shift + H or Command-Shift-H).
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In Sibelius 1 (and as an alternative in later versions of Sibelius), to switch the sustain pedal on, insert the text object ~C64,127 in your score. To switch the pedal off, insert ~C64,0. For further information on the MIDI messages you can insert in your score, read this PDF (95K, required Adobe Acrobat Reader).