Before obtaining medical care, you must enroll in TRICARE Prime or transfer your current Prime enrollment to Fort Sill. TRICARE Prime enrollment is mandatory for all Active Duty Service Members who are stationed at Fort Sill for six months or longer. TRICARE Prime enrollment is voluntary for family members and retirees and their family members. You can enroll at the TRICARE Service Center (1st floor, south entrance of the hospital) or online at www.humana-military.com.