First, make sure you've correctly installed the Sympli Handoff extension in your organization.
Linking a Sympli Screen to Azure Boards
From your project "Designs" tab, select an artboard (or multiple) using the checkmark located in the top right corner of the preview. You can also use the standard shift+click keyboard shortcut, which is quicker. After that, click "Link to Azure Boards".
After that, begin typing the name of the issue that is currently being tracked in Azure Boards, and then select the issue from the list of results returned in the box. You can link screens to as many tracked issues as needed, as normally, there'll be numerous issues to track on different aspects of the same screen.
If you're part of several organizations and/or projects, you'll also be able to easily see where you're linking designs to, and switch between them by clicking on their name.
Artboards with linked issues will be assigned a tag with the issue name, which allows you to easily filter and find what you're looking for.
Tracking an Issue in Azure Boards
Here's the actual issue as it appears in Azure. You'll see a list of the linked designs in the Summary, along with a link to the actual design in Sympli.
The Sympli tab in the right-hand menu will also show a preview of these artboards, so everyone on your team can have the context they need.