Greetings!
· You cannot import .bak file directly in Azure SQL database. Import/Export feature of Azure SQL database support .bacpac file.
· Instead of taking a backup you can export the database to .bacpac file.
· After this you can import the .bacpac file to create an Azure SQL Database either through SSMS or through the Azure Portal.
Please refer the below links to export database as .bacpac file: Export a Data-tier Application - SQL Server | Microsoft Learn
Import Bacpac file using Azure Portal: Import a .bacpac file to create a database in Azure SQL Database - Azure SQL Database | Microsoft Learn
upload the bacpac file in storage account in Azure Portal to import using the Portal.
Import Bacpac file using SSMS: Import a BACPAC file to create a new user database - SQL Server | Microsoft Learn
Regards
Geetha