Tirthankar does not say “Namo Arihantanam” before taking Diksha.

Reason in simple words:

• Before Diksha, the person is still a shravak (householder), not Arihant, not Tirthankar.
• Navkar / Namokar Mantra line “Ṇamo Arihantāṇam” means:

> I bow to the Arihants – those who have already destroyed inner passions and attained keval‑jnan (omniscience).

• A future Tirthankar, before Diksha, is only a Tirthankar‑nam‑karma wali jeev (has the special karma to become Tirthankar later). At that time he is spiritually a sadhu-to-be, not yet Arihant.
• So he recites Navkar as a sadhak, bowing to existing Arihants and Siddhas, not to himself.

Later sequence in brief (traditional Jain view):

1. Diksha – complete renunciation, becomes a great monk.
2. Keval‑jnan – becomes Arihant / Jina / Tirthankar.
3. From then on, others say “Namo Arihantanam” to him; he himself remains absorbed in pure knowing & seeing, beyond our kind of verbal stuti.

About “Namo Arihantanam” and why it is first in Navkar Mantra is explained nicely here: “Understanding the concept of Navkar Mantra (The most fundamental Jain prayer)”