code:

//put your connection strings in your appSettings.json like this:
"ConnectionStrings": {
  "TenantADataModel": "data source=<rest of connection string here>;",
  "TenantBDataModel": "data source=<rest of connection string here>;"
},

 public partial class MyDbContext : DbContext
 {
     private readonly ITenantIdentifier tenantIdentifier;
     private readonly IConfiguration configuration;

     public MyDbContext(IConfiguration configuration, ITenantIdentifier tenantIdentifier)
     {
         this.configuration = configuration;
         this.tenantIdentifier = tenantIdentifier;
     }

     protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
     {
         var connectionString = configuration
             .GetConnectionString(tenantIdentifier.GetCurrentTenantId() + "DataModel"); 
         optionsBuilder.UseSqlServer(connectionString);
     }	 
}

in Startup.cs:
 
services.AddSingleton<ITenantIdentifier, MyTenantIdentifier>();
services.AddDbContext<MyDbContext>();