Migrating Azure SQL database to Azure sql Managed instance
The only way to go with this to get the bacpac file for your Azure SQL Database. Then import this bacpac file into your Managed Instance. This can be achieved through :


SSMS 
Export\import using sqlpackage.exe
Via portal which appears to be a bit slow as the Azure SQL Database Import/Export service provides a limited number of compute virtual machines (VMs) per region to process import and export operations
SSDT 
bcp


This link explains each method : Export bacpac

Once done. Import the bacpac file into your managed instance using SSMS,sqlpackage or portal.

This will walk you through the process : Import bacpac