As you probably know, Belgian taxpayers must now communicate the accounts they have opened abroad to the central contact point (PCC) managed by the National Bank.
In that framework, at the latest at the same time as the introduction of your income tax return, you will have to communicate your Birdee account which is opened in Luxembourg to the CCP.
It should be noted that you must also mention the existence of your account invested with Birdee in your annual tax return.
It is worth remembering here that a hundred or so countries committed to exchange a great deal of banking and financial information every year (see CRS, Common Reporting Standard, developed and promoted by the OECD). The Belgian administration will thus know the values held in Luxembourg and in many other countries by all Belgian residents.
In this context, Birdee must declare annually all the portfolios held by its customers as of December 31st. This reporting will be done via your TIN tax identification number, ie in Belgium, your national registry number.