CustomUser.cs

// .NET C#
var mobileServiceClient = new MobileServiceClient("<your-app-url>", "<your-app-key>");
mobileServiceClient.CurrentUser = new MobileServiceUser("Foo:123456789");
mobileServiceClient.CurrentUser.MobileServiceAuthenticationToken = "<your-users-JWT>";

CustomUser.js

// Javascript
var client = new Microsoft.WindowsAzure.MobileServices.MobileServiceClient(
    "<your-app-url>",
    "<your-app-key>");

client.currentUser = {
    userId: "Foo:123456789",
    mobileServiceAuthenticationToken: "<your-users-JWT>"
};

CustomUser.m

// Objective C
MSClient *client = [MSClient clientWithApplicationURLString:@"<your-app-url>"
                                         withApplicationKey:@"<your-app-key>"];

client.currentUser = [MSUser alloc] initWithUserId:@"Foo:123456789";
client.currentUser.mobileServiceAuthenticationToken = @"<your-users-JWT>";