This prestigious competition is for high school students and applications are not done individually. Each country has a Mathematical Olympiad Committee that selects participants through a series of national exams.  You can't directly apply to the IMO itself.Many countries hold Math Olympiads at various levels to select participants for the IMO.  These might be called IMO or have different names.  The process for applying will vary depending on your country.

Look for "[Country Name] Math Olympiad" or "[Country Name] Mathematical Olympiad Committee". The official website will have details on eligibility, application process, and important dates.
They might be familiar with Math Olympiad opportunities and can guide you on how to participate.

https://school.careers360.com/exams/imo

I hope it helps!