There are two ways to add reports to a dashboard:

You can pin any report to an existing dashboard, via the menu on that report.
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This will open a popup, where you can select which dashboard you’d like to pin the report to. Note that, any chart-level selections (e.g., Classifiers, Topics) will be preserved when pinning to a dashboard.
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Within any custom Signals Dashboard, you can click +Add Report in the top right side.
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This will open a popup, where you can select which reports you would like to add to your custom dashboard.