The affix ṭhak comes after the word dharmaṃ being in the Accusative -2nd case in construction in the sense of 'who practices that'.,
Thus धर्मम् चरति = धार्मिकः \religious\. So also आधर्मिकः \irreligious undutiful\, by a Vartika.,
धर्मं चरति
Adhyāya 4 · Pāda 4 · Rule 41