Apply the screen native screen res and refresh in your OS. You have to out the OS Dispkay Mode to Extended not Duplicate.

If res is lower then natuve, some screens you will have to enter the display on screen menu and choose the option to stretch to fit

winkey + p, pick extend, config the res and move whatever you want to display on it

if the laptop display and external display are the same res/refresh you can pick clone

Stretch to Fit is usually something you'd more often find on a TV.
Most Monitors have this enabled by default in it's menu sub settings (OSD)
If the TV is 4K and you are doing Duplicate when your primary Display is say; 1920x1080 then it might not fit the borders of the TV until you use the TV Remote and go into the settings and change it so it has "Screen Fit" enabled

When PC is set to Extended; then you can set each Display to a # and set each to their own Screen Res + Refresh Rate. Then treat all screens as a one big desktop area.

if its a tv, go to the tvs menu and disable overscan, may be just fit or native or something like that, every mfg calls it something different
then instead of a lower res scaled up by the tv, its 1:1 pixel ratio

It is nothing to do wit a tv. i have connected a standard compute monitor to my Laptop. when i start the laptop the monitor displays the same size picture as the Laptop. It does not fill the entire screen of the monitor.There is a large black area around the picture that is unusable. i want the picture to fill the monitor screen like it does on a monitor connected to a PC
If any one knows how to get this to work can you give me full instructions please as i have not used a Laptop before.
It is like when you get the picture on you PC to show in window mode.
Sorry if i am nor explaining this well.

right click on desktop and change res, set the external display to native
and set display scaling to 100%

All okay now i enabled HDR and the External Monitor is now displaying correctly

thanks all for the help

Lasted until i accidentally shutdown my Laptop. now back to same problem

Why HDR? HDR is 100% crap

Just configure the screen res + refresh properly.

If you want the colors, brightness, contrast and such to look better, adjust the Monitor's settings via OSD menu system