To make an external link open in a new tab, there may be many option to do that but here is the easiest one.
Go to the any content page, text editor. Type your content here and select the part of the content that you would like to give an external link. In the below picture you can see the selected part of the content. Select your content then go to the Link icon from the WYSIWYG filtred.
Now, you will see the screen pictured below. Here you need to add your URL.
Copy and paste your URL link here. Click on the Target, next to the Link Info showed in the picture.
Select the New Window option from the drop-down list of the Target. Click OK.
Save the page and when you will click on the link you will see the result as opening it in a new tab.