If you came to work in the Netherlands this year and received the 30% ruling (expatregeling), you file your M-form with the taxable salary as shown on your annual statement: your employer already applied the tax-free allowance there. In your immigration year, the maximum tax-free allowance is calculated proportionally (tijdsevenredig) over the period you actually used the ruling.
The ruling applies to employees recruited from abroad who, in the 24 months before their first working day, lived more than 150 kilometres from the Dutch border for at least 16 months, with a taxable salary above the statutory salary norm. Employer and employee apply for the ruling together at the Belastingdienst.
You enter the taxable salary as shown on your annual statement; your employer has already excluded the tax-free 30% allowance from it. In your first, partial year the maximum allowance is proportional to the period you used the ruling that year.
The wizard uses the amounts from your annual statement, including the 30% ruling already applied, and fills in the rest of your migration year: your own home, a fiscal partner if you have one, and box 2 or box 3 where they apply.