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 memoranda. You can read the arbitration decision which upheld this right.
If you need to copy/paste the preamble for your memo, here is the full text:
On (date) at (time) at (place), I was ordered to make this statement by (name/rank of individual). I submit this statement at (his/her) order as a condition of employment. (Agent/Union Representative Name and Title) witnessed me receive the order. In view of possible discipline for insubordination, I have no alternative but to abide by this order.
It is my belief and understanding that the agency requires this statement solely and exclusively for internal purposes and will not release it to any other agency. It is also my understanding that this statement, since compelled, will not and can not be used against me in any criminal proceeding. I authorize the release of this statement to my attorney or designated union representative.
I may amend or correct any portion of this statement upon reflection to correct any unintended mistake without subjecting myself to a charge of untruthfulness.
For any and all purposes, I hereby reserve my constitutional right to remain silent under the fifth and fourteenth amendments to the United States Constitution and any other rights prescribed by law.
I specifically rely on the protection afforded to me under the doctrines set forth in Garrity v. New Jersey, 385 U.S. 493 (1967) and in Spevack v. Klien, 385 U.S. 511 (1967) in submitting this statement.
(Start text of statement here)