For idam there is the substitute iś when a prāgdiśīya affix follows.,
The word प्राग्दिशः is understood here also. The श् of इश् is indicatory of the substitution of the whole (1. 1. 55). Thus इदम् + ह 5.3.11 = इह ॥,
इदम् इश्
Adhyāya 5 · Pāda 3 · Rule 3