Out of box, there is no easy way to do this. The fastest would be to create a new node type such as a Category page in the community structure where you want it go to. Leave it as hidden for now. Contact Support to create a redirect from that URL to the external URL you want it to go to. Then unhide the URL in structure. When users click on it, it will go to wherever you have Support set it up to go.
Other options:
1 - You can use a quick script to redirect automatically such as this which will cause the page to redirect as soon as it loads (not best experience)
<meta http-equiv="refresh" content="0; URL=https://www.google.com/" />
2 - Some communities have build a redirect tool where you can easily redirect any node directly in Admin to an external page. This requires development work on your end, but can pay off easily if it is used a lot
