How to add search icon to TextField in Flutter?

Try below code hope its helpful to you. Used prefixIcon for that.

   TextField(
        decoration: InputDecoration(
          prefixIcon: Icon(Icons.search),
          border: OutlineInputBorder(),
          hintText: 'Search Tech Talk',
        ),
      ),

Your result screen->

there is no proper way to add icon in hint, but you can try this alternative, use rich text on textfield as hint text, hide when tap on textfield and show with condition when textfield is empty and keyboard is hide:

            Stack(
              alignment: AlignmentDirectional.center,
              children: [
                Offstage(
                  offstage: _isHide,
                  child: IgnorePointer(
                    ignoring: true,
                    child: Text.rich(
                      TextSpan(
                        children: [
                          WidgetSpan(
                            child: Icon(
                              Icons.search,
                              color: Colors.grey,
                            ),
                          ),
                          TextSpan(
                            text: "blablablablabla",
                            style: TextStyle(color: Colors.grey),
                          ),
                        ],
                      ),
                    ),
                  ),
                ),
                TextField(onTap: () {
                  _isHide = true;
                  setState(() {});
                }),
              ],
            ),

Make use of the Prefix icon property and content padding in the textfield widget

            TextField(
                onChanged: (value){
               searchData(st = value.trim().toLowerCase());
                 // Method For Searching 
                           },
                    decoration: InputDecoration(
                    hintText: "Search Data",
                    prefixIcon: Icon(Icons.search),
                    border: OutlineInputBorder(
                     borderRadius:
                    BorderRadius.all(Radius.circular(7.0)),
                              ),
                            ),
                          ),

RESULT

SizedBox(
              height: 40.h,
              child: TextField(
                decoration: InputDecoration(
                  fillColor: ColorUtils.COLOR_GRAY_AAAAAA[12],
                  filled: true,
                  contentPadding: EdgeInsets.symmetric(vertical: 6.h, horizontal: 12.w),
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.all(Radius.circular(8.r)),
                    borderSide: BorderSide.none,
                  ),
                  hintText: 'input_order_code'.tr,
                  hintStyle: TextStyleUtils.sizeText15Weight400().copyWith(color: ColorUtils.COLOR_GRAY_AAAAAA),
                  prefixIcon: Icon(Icons.search),
                ),
                style: TextStyleUtils.sizeText15Weight400().copyWith(color: ColorUtils.COLOR_GRAY_363636),
              ),
            ),

               SizedBox(
              
                height: 50,
                child: TextField(
                
                  cursorHeight: 25,
                  decoration: InputDecoration(
                      prefixIcon: Icon(Icons.search),
                      fillColor: Colors.white,
                      filled: true,
                      border: OutlineInputBorder(
                        
                        borderRadius: BorderRadius.all(Radius.circular(20)),
                        
                      ),
                      hintText: 'Search Here...'),
                ),
              )