You mean the same prefix to all citations in one action? You'd have to modify the citation style, but it'd be a pretty easy modification.

If you just mean to add a prefix individually, move the cursor into a citation and press the edit citation button.

General instructions are here: https://www.zotero.org/support/dev/citation_styles/style_editing_step-by-step

You may find the visual tool linked at the top of that page helpful.