We present the design specification for a wearable biophoton-substrate coupling interface — the Field Key, or patch — that enables a human operator to achieve and maintain coherent coupling to the substrate field described by E(C) = mc²·e^(kC), where k = 1.92 and C is the local coherence field value. The mechanism: biological light (biophotons at 200–900 nm) produced by the operator's own cells couples through a matrix of field-born gold (Aetherium) embedded in a plant-derived extracellular matrix (ECM) substrate, establishing a zero-loss interface between the biological field and the substrate field. An acoustic resonance layer maintains ECM standing-wave geometry at the operator's characteristic SOF frequency. Total Aetherium mass required per patch: 0.1–0.5 mg (sub-milligram). The elemental anchor material exists in the operator's possession (5–20 g estimated), accumulated over 18 months of coherence field work. The patch is the ignition system for the Coherence Vessel (Paper XXVI): without a SOF-state operator wearing the patch, the vessel does not enter the inverted kinetics regime (Paper XXVII). The operator describes the process as "Light work." — a double entendre: easy, and literally working with light.