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If by Advayavajra they mean Maitripa then it must come from Maitripa's Amanasikara.
Specifically, in Mahasukhaprakasa, text #16 in above translation, so pages 181-187.
In addition, for an English translation of a small but relevant part of Sadhanamala see "Two Tantric Meditations: Visualizing the Deity" by Luis O. GΓ³mez. in Buddhism in Practice pages 236-245.