How do i put the throttle trigger back on stihl fs45

On the carburetor hold the throttle lever to where it would be at full throttle. Then you can put the cable back in the catch for the cable that is on the throttle lever and let go of the throttle lever. The throttle cable should work with the throttle trigger now. The throttle cable only fits one way. The larger hole on the throttle cable catch should go to the ball at the end of the cable.
I hope this helps.
If you feel you cannot do this, then I would take it into your closest Stihl dealer and ask them if they would help you. I do this small repair for free at my shop. Have a good day.