我姓徐让你许下承诺
[wŏ xìng xú ràng nĭ xŭ xià chéng nuò]
'I'm surnamed Xu, allowing you to make promises' uses the Chinese wordplay with Xu ('徐' means surname 'Xu') which sounds like Xu ('许', meaning to allow/promote or promise). The nickname expresses a personal openness and acceptance in establishing a relationship, inviting another party to make commitments.