Based on standard possessive rules for singular nouns ending in "z," the proper text is . Grammar Rule: Add an apostrophe followed by an ('s) to singular nouns ending in to show possession [Source 1, Source 2].
Is this regarding a specific name, a character, or a possessive in a novel? If you tell me the context, I can confirm if "Schwartz'" (just the apostrophe) is preferred by a specific style guide (like AP Style) or if "Schwartz's" is better for your usage. Schwartz' wing
Schwartz's wing, Delmore Schwartz's poetry, Ms. Schwartz's car. Based on standard possessive rules for singular nouns