I suspect that rather than board screenshot you mean how to copy a chess position from the board in Chesbase or Fritz and post it on here ? Ok that is very straightforward here is how you do that:
In Fritz (or CB) once you have the position on the board go to HOME
tab then click on COPY POSITION
Now on here in the forum when you want to post that position you click on the FEN POSITION
icon above the text box and paste in the FEN you just copied from Fritz. It will look like a random string of characters but don't worry:
Now when you post that (or do a preview) it will actually display the chessboard image like this:
If on the other hand you do actually mean literally screen capture, or you want to post any image of anything other than a chess positio, then just use any free screen capture tool or the one already built in to windows (search for "snipping tool" in your windows and you will find it). Capture the screen image you want and the save the file somewhere.
Then use any image hosting site. I use this one https://postimage.io/
which is good and easy to use. Upload your saved image and choose "direct link".
Now in your forum post click on the POST IMAGE
icon and paste in that "direct link" url from the image hosting site and click INSERT like this:
The image will then appear when you post or do preview (by the way thats is how I captured and posted the screen shots to make this guide).
Hope this helps.