Startup.cs

services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
services.AddDbContext<ApplicationDbContext>((serviceProvider, dbContextBuilder) =>
{
    var connectionStringPlaceHolder = Configuration.GetConnectionString("PlaceHolderConnection");
    var httpContextAccessor = serviceProvider.GetRequiredService<IHttpContextAccessor>();
    var dbName = httpContextAccessor.HttpContext.Request.Headers["tenantId"].First();
    var connectionString = connectionStringPlaceHolder.Replace("{dbName}", dbName);
    dbContextBuilder.UseSqlServer(connectionString);
});