Vaishnava Samhita. Shri Vyasadasa Charitam I. Vol II Book VI Chp 2 Shloka 9

दासीभूत्वा राधिकायाः सेवां कुर्वन् यथारुचि 

रसिका भावुको भक्तः कृष्णसङ्गममाप्नुयात् 

A prema bhakta who is a true rasika alone, unites with Shri Krishna becoming the  maid of Shri Radha and serving Her according to taste.