Short answer: Yes – as per Jain siddhānt, a bhavī (future) Tirthankar does have bhakti towards Siddha Parmeshthi and recites the Navkār Mantra (which includes “Namo Siddhāṇam”) before dīkshā. But scriptures do not say that there is a special rule that “he must say only ‘Namo Siddhāṇam’ just before dīkshā.”

A bit more clearly:

1. Order of respect (Pañch Parmeshti)

- Arihant - Siddha - Āchārya - Upādhyāy - Sādhu A Tirthankar-to-be (while still a human) has deep devotion to all five, including Siddhas.

1. Navkār Mantra

The Navkār Mantra contains: - “Namo Arihantāṇam” - “Namo Siddhāṇam” - “Namo Āyariyāṇam” - “Namo Uvajjhāyāṇam” - “Namo Loe Savva Sāhūṇam” So whenever he recites Navkār, he is definitely saying “Namo Siddhāṇam.”

1. Before dīkshā

- Before taking dīkshā the future Tirthankar is a gṛhastha (householder) with samyak darshan and very high-level spiritual virtues. - Such a soul naturally does Namokār jāp, stuti, vandanā of Pañch Parmeshti. - In many traditional kathās, before dīkshā, the Tirthankar does Namokār Mantra and offers mental obeisance to all Pañch Parmeshti. - However, śāstras do not specifically say: “He must say this exact line ‘Namo Siddhāṇam’ at the moment of dīkshā and nothing else.” The emphasis is on overall Namaskār to Pañch Parmeshti, not on one line alone.

1. After nirvāṇ

Once the Tirthankar attains Siddhahood, there is no recitation at all, because a Siddha is completely beyond activity (nirvyāpār).

So:

• Yes, a future Tirthankar certainly has devotion to Siddhas and recites “Namo Siddhāṇam” as part of Navkār before dīkshā.
• But there is no separate, special ritual rule in Jain scriptures that only “Namo Siddhāṇam” must be said right before taking dīkshā.