When you’re logging on you will see the below tab circled in red.

Fig 1

Second Screen1

Once your desktop has loaded click on the black tab as above.

You will see the option as per below in red relating to your monitor.

Fig 2

Second Screen2

Click on this to minimise your Citrix screen, now click your mouse on the top bar and drag the whole.

Window so that its half on the working screen and half on one that’s not as in Fig 3.

Fig 3

Second Screen3

Now click the tab again at the top of the screen as in Fig1. Click the screen option again and make it full screen, this should now enable the second screen for you.