You can find this number on your Belgian identity card, a LIMOSA-notification, previous tax returns or in most cases on personal documents from the Belgian authorities.
Please provide, the summary statements and play slips of all the employment income earned outside Belgium in 2021. Please note that such income is not taxable in Belgium, but we require this information to determine whether you are eligible for the personal tax allowance and other related deductions. Some further explanation on this you may find on this page:
https://www.taxpatria.be/faq/what-is-the-75-rule-for-non-resident-taxpayers/Please indicate the amount, the beneficiaries and address where your children/ex-spouse is/are residing.