(By Clicking this button the link will open in a new tab)
For that first add a link by clicking "Link" in your blogger editor.
Or if you are doing HTML editing you can use this code below along with your hyperlink.
This will only open links in a post only.
To make all links to open in a new window follows these steps.
Go to your Blogger Dashboard.Go to template option and click "edit HTML".
Now find the <head> tag by pressing Ctrl+F and add the code below <head>.'
Save the template and you are done.Now all your links will open in a new tab.