Vibrato - Guitar

I guess you are talking about acoustic/classical guitar in standard notation.
Commonly, vibrato is indicated with a verbal instruction (ordinary text, abbreviation usually in italics), ie: vib.

Alternatively (according to your purpose), you can use the symbol "Guitar shake": press "Z", shortcut for Symbols palette -> type "guitar" in the search box: the symbol is the first one, sort of wavy line.

But I recommand the text. See image below:

If you are talking about electric guitar, the vibrato is commonly nammed "tremolo bar": see the "v" in Articulations & Ornaments palette.