From Cancellation Services, Stamp Fulfillment Services, U.S. Postal Service:

“In an effort to assist our Santa Clarita Post Office, Cancellation Services will be receiving and processing all requests for the 2/18/2021 Perseverance on Mars Station pictorial postmark as show in last week’s PB & seen below.”“All requests already sent to Santa Clarita will be forwarded to SFS for processing.”

However, you can now send postmark requests directly to:

USPS SFS Cancellation Services
8300 NE Underground Drive, Pillar 210
Kansas City, MO 64144-9998