There is no one fixed sentence that every Tirthankar says before taking Diksha.

In Jain scriptures, for each Tirthankar, the situation and words are described differently, but the bhāva (inner feeling) is the same:

• Complete vairāgya (detachment from kingdom, family, body, pleasures)
• Firm nishchay (decision) to attain Keval Jnān and then Moksha
• Deep karunā (compassion) for all living beings

Some simple examples:

• Bhagwan Neminath: On hearing the crying of animals kept for slaughter at his wedding, he develops intense compassion and says (in essence):

“How can I enjoy happiness built on the suffering of these innocent beings?” And then he renounces and takes Diksha.

• Bhagwan Mahavira (Shvetāmbara tradition):

He first takes permission from his parents (or after their death, from elder brother Nandivardhan), expresses his desire to renounce, gives away his wealth and then takes Diksha. (In Digambar tradition, he leaves silently without asking permission.)

Across all Tirthankars:

• Before Diksha they give up: kingdom, ornaments, family ties.
• They decide internally:

“Now I will walk only on the path of right faith, right knowledge and right conduct, until I destroy all karmas.”

So, scriptures do not give one standard line that every Tirthankar “says” before Diksha, but they clearly show their inner decision of total renunciation and compassion, which is the real essence.