How to send SMS from terminal in background on Android?
The syntax that appears to work as attested over at our sister site stackoverflow is and is explains in detail:

adb shell service call isms 7 i32 0 s16 "com.android.mms.service" s16 "+1234567890" s16 "null" s16 "Hey\ you\ !" s16 "null" s16 "null"


  "Isms method number (5 and 7 above) may change with the android version . Read full explanation to understand it."