When cross assigned to a unit that is a surface warfare community, how do I go about earning the Surface Warfare Pin?

I would say to start asking within your Naval Reserve unit...but a Quick Google Search result shows:

Key Steps for a Reserve YN:
Get Assigned to a Warfare-Qualified Unit: The most crucial step is finding an assignment (e.g., on a ship, with a specific unit like NECC, or a seafaring command) that actively supports and qualifies personnel for warfare pins.
Join the Warfare Program: Once assigned, volunteer or enroll in the command's Surface Warfare program, often managed by the ship's training department.
Complete PQS: Work through the Performance Qualification Standards (PQS) for your rating (YN) and general shipboard requirements, learning various stations, engineering fundamentals, damage control, and watchstanding.
Get Sign-Offs: Have experienced personnel sign off on your PQS book as you demonstrate proficiency in each area.
Pass the Boards: You'll face multiple boards, starting with division officers, then department heads, and finally a culminating board with the Commanding Officer (CO) and Executive Officer (XO), answering detailed questions.
Qualify: Upon successfully completing the final board, you'll be authorized to wear the Enlisted Surface Warfare Specialist (ESWS) pin.
Important Considerations for Reservists:
Assignment is Key: Your primary opportunity lies in being assigned to a sea-going command or a unit with an active SW/ESWS program.
Time & Commitment: Earning the pin requires significant time and dedication, often months of study and on-the-job training, which can be challenging but achievable with focused ADT or deployments.
YN Role: As YNs serve in various domains, you must qualify based on your assigned command's warfare type, but the surface warfare path is a classic route for this rating