You did not indicate what device you are using so I am assuming you are using Windows Phone 10
There are two choices to use. Either Skype or the Microsoft Messaging app. I choose to use the Messaging app just because I like it a lot.
Go into Settings/System and click on Messaging. Under Default apps, click the drop-down and select Skype or Microsoft Messaging.
I'm not sure if this is what you are looking for so if there is an additional question just ask.