This is not always quite as easy as the software help files may suggest and often for ease of use and control of RF breakthrough to the rig or computer some further interfacing work may need attending to !
Many people make do with a homemade switching arrangement arranged so the sstv tones are transmitted only when the microphone is connected using a double pole switch to achieve this
You will find often some more sophisticated set up will make life easier and sstv more enjoyable.
Suitable interfaces are available commercially and a few links are on my link pages .
I personally have used until recently a home made relay switched unit but recently as I have been using more complex modes I am having good results using an opto isolation unit from Dave G3VFP (see links)
As I run a linear I have had to add a few ferrite rings to eliminate RF break through to my computer mouse ... would you believe !!!