How do I indicate that material in square brackets is part of the original text?
Normally, when you quote from a source, your reader is to assume that any styling, such as italics or punctuation in the quotation, is in the original text. But, as the MLA Handbook (sec. 1.3.6) explains, square brackets are used to indicate material you have added. Thus, if square brackets are in the original, provide an explanation in parentheses after the quotation, as shown in the following example:
“He wanted to have [his] cake and eat it too” (square brackets in the original).
MLA Handbook. 8th ed., Modern Language Association of America, 2016.