If you want the chicken to be browned chicken before putting it in the slow cooker then you should brown the chicken before slow cooking it.
But if you just want to cook the chicken and not have it browned then there's no need to brown the chicken or cook the chicken before putting it into the slow cooker.
The slow cooker will slowly cook the chicken and it will be fully cooked after 8 hours or so.
I just place chicken or any other meats into my slow cooker and go do some shopping, go to work, go to sleep etc and then when I wake up or get back the food is ready to eat.
Never have I cooked or browned any chicken or any other meats before placing the meat or chicken into a slow cooker.
The slow cooker does all the work of cooking it.
However if you do want to brown the chicken before cooking it in the slow cooker then you can do so.
The slow cooker won't brown the chicken so if you like your chicken browned then you need to brown it.