First of all it is लेख्यमान: where लिख् + ण्यत् = लेख्य then लेख्य + शानच् = लेख्यमान: पुल्लिङ्गे = one who is writing currently.
Once you use this form in karmani other auxiliary form like अस् will be used in present tense only.
Meaning of your sentence = What is being written by you?