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Correction: It needs to be in Ambient mode. Either Incident or Reflected measurements can be made in either Ambient or Flash mode, but as mentioned Flash mode is always aperture priority.
You can set a standard shutter speed in flash mode, but that's not the same as Shutter Priority mode.
Ambient / Shutter Priority mode is covered on Page 12 of your manual, and setting the shutter speed for Flash mode is on Pages 17 &18: