Create and Send a Text Message - HTC One® (M8) for Windows®
From the Start screen, swipe left to display the Apps list.
Alternatively, tap All apps (located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap new (located at the bottom).
From the 'To' field, enter a 10-digit mobile number.
Alternately, enter a contact name. As the name is entered, a matching contact may appear and can be selected at any time.
Alternately, tap the Plus symbol (located in the upper-right), tap a contact then tap the appropriate mobile number (if applicable).
Repeat this step as necessary to add additional recipients.