When the prologue—or author’s preface—to a play is in prose, provide a page number for a quotation from or reference to the work if it is given.
“Money is the most important thing in the world,” declares Shaw in his prologue to Major Barbara (19), as if we didn’t already know that.
Shaw, George Bernard. “First Aid to Critics.” Major Barbara, Brentano’s, 1917, pp. 5–48.
No number is needed if the work is online or otherwise available electronically.
In defiance, Shaw describes his play as “intensely and deliberately didactic.”
Shaw, George Bernard. Preface. Pygmalion. Project Gutenburg, 19 Jan. 2005, www.gutenberg.org/app/uploads/sites/3/3825/3825-h/3825-h.htm.
If the prologue is in verse, you have the option of providing a line number instead of a page number.
In the play’s prologue, Shakespeare tells his audience upfront how long Romeo and Juliet will take: “the two hours’ traffic of our stage” (line 12).
Shakespeare tells his audience in the prologue that in Henry V Harry is now “warlike” (line 5).