Answer

In Jain tradition, before starting Māsakṣama or any tapas, the most commonly recited mantra is the Navkar Mantra.

Explanation

There is no single compulsory mantra for every Jain sect, but generally:

• Navkar Mantra is recited first
• Then one takes sankalp with a pure mind
• Many people also do it with the guidance of a Guru Bhagwant or according to their local tradition

A simple beginning is:

• Recite Navkar Mantra
• Pray for forgiveness
• Take the vow of Māsakṣama with humility and devotion

Spiritual Understanding

The purpose of chanting the mantra before tapas is to:

• purify the mind
• remember the Arihants, Siddhas, Acharyas, Upadhyayas, and Sadhus
• begin the vow with श्रद्धा, humility, and right intention

In Jain Dharma, the inner भावना is more important than only the words.

Takeaway

Before Māsakṣama, recite the Navkar Mantra with श्रद्धा and then begin the vow in a pure and शांत mind. If you are following a specific Jain tradition, it is best to follow that exact vidhi.