The Garrity preamble is the formal assertion of certain rights which are protected by law.
Border Patrol agents do not lose their constitutional rights just because they work as law enforcement officers. Agents have the right to include this statement in their memorandum. If you need to copy and paste the preamble for your memo, here is the full text:
On (date time), (xyz sup) requested I submit a memorandum regarding a (incident). I am therefore complying with (xyz's) request as a condition of employment. However, the content of this memorandum and the statements contained herein are protected under the Fourteenth Amendment and Fifth Amendment in light of the decision in Garrity v. N.J., 385 U.S. 493. Thus, the facts described herein reflect the events to the best of my recollection at this current time.