The payment the form allows is the housing share you set, times gross monthly income, minus the monthly debts you enter. That payment is turned back into a loan amount with the standard mortgage formula at the rate and term you enter. Adding the down payment gives a purchase price.
A household that can put $2,200 a month toward principal and interest, at 6.2% for 30 years, supports a loan near $360,000. With $90,000 down, the price is near $450,000. Change the rate or the term and both numbers move.
This is not a lender pre-approval. It is not the Canadian mortgage stress test. Taxes, insurance, and mortgage insurance are outside the payment unless you have already reduced the housing budget to leave room for them.