WHERE DO I WRITE MY LED SMART TV CODE, IN ORDER TO HAVE THE SCREEM AUTOMATICALLY SET FOR FACEBOOK?

Bad backlights

You can just enter its detail anywhere by internet. Wherever you enter isn't matter. It will go straight to Hisense; China.

Does your TV have RJ45 connector ?
That looks like
Insert a RJ45 patch cable from the router to that port using taht looks like this

Your TV should AUTOMATICALLY now be on the network

If you type "showmax on hisense smart TV" in google it will give you several sites that tell you how to set it up. One claims to only take 2 minutes. Give this a try.

No need to try to set it up; without a remote control.

Ok what kind of tv do you have? I need the make model ? And what kind of remote do you have?

Set-up (Programming) procedure is too much to type in here, and is not practical and it will differ brand-to-brand remote controls. If you wish to get more; check the sites linked here. It has set-up code list & set-up procedure to many popular brand remote controls. One among it has general set-up codes to check with TVs and other devices are given, in the order of the 'device's brand name list' in alphabetical order. Pull up older posts. Search the sites by type in the brand name to your device in its search box at the top right part of the display window. It will help you to find the document easily.
http://electro-medical.blogspot.com/

Set-up (Programming) procedure is too much to type in here, and is not practical and it will differ brand-to-brand remote controls. If you wish to get more; check the sites linked here. It has set-up code list & set-up procedure to many popular brand remote controls. One among it has general set-up codes to check with TVs and other devices are given, in the order of the ‘device’s brand name list’ in alphabetical order. Pull up older posts. Search the sites by type in the brand name to your device in its search box at the top right part of the display window. It will help you to find the document easily.
http://electro-medical.blogspot.com/

Hi,
here's the spec's for the 50" Model, it shows a headphone output and it will certainly be a 3.5 Jack: HISENSE LTDN50K321UWTS 50 inch 4K Ultra HD Smart LED TV Freeview

0840.0924.
Try these codes. If you wish to get more details; check the sites linked here. It has set-up code list & set-up procedure to many popular brand remote controls. Common codes to check devices with different brand TVs are given, in the order of the device's brand name list in alphabetical order. Pull up older posts. Surf the site with patience.
http://remotesearch.blogspot.com/
http://electrohelponline.blogspot.com/