Please take notice that the mobility of work force is restricted at the moment. For more information, check out the FAQ site at the Ministry of Economic Affairs and Employment of Finland here, check this information also if you are already in Finland.
The right to work in Finland is determined by one’s nationality. An employee who is not a citizen of an EU country, Iceland, Norway, Liechtenstein or Switzerland usually needs either a residence permit for an employed person or another type of residence permit which entitles for gainful employment. Some tasks specified in the Aliens Act may be performed for a limited period of time with a visa or fully exempted from visa.
More information on different types of permits on the website of the Finnish Immigration Service.
If you are applying for seasonal work (link):
The types of seasonal work for which a permit is required include forestry, crop farming and festival work. A detailed list of seasonal jobs. If you work as a forest berry picker, you do not need to apply for a permit if you will work for less than three months.
You can apply for work permits in the online service of the Finnish Immigration Service: https://enterfinland.fi/eServices
How to use the service:
Basic information on working in Finland (different language versions):