Guhya means secret and Guhya Ṣoḍaśī is one of the most secretive mantras and is not widely practiced. The following mantra appears in Mantramahodadhiḥ (taraṃga 12). There is no reference to this mantra in Mantramahārṇavaḥ. The Pañcadaśī mantra of Lopāmudra is used while formulating Guhya Ṣoḍaśī.

ॐ ह्रीं ॐ श्रीं ह्रीं
सौः क्लीं ऐं
ह स क ल ह्रीं
ह स क ह ल ह्रीं
स क ल ह्रीं
ॐ ह्रीं ॐ श्रीं ह्रीं

om hrīṁ om śrīṁ hrīṁ
sauḥ klīṁ aiṁ
ha sa ka la hrīṁ
ha sa ka ha la hrīṁ
sa ka la hrīṁ
om hrīṁ om śrīṁ hrīṁ

(The authenticity of this mantra is not verified with other sources, as no such source is available to me. This mantra is published as someone sought to know about this mantra, which has been published as a comment in the article Mahāṣoḍaśī Mantra.)

Variations of SHODASHI MANTRA is given in this posting.