Hi - There are two ways you can add assets to the panel. In the Libraries panel, select a symbol you want to convert to asset. Right-click the asset, and then select either Export Asset or Save As Asset option.
Export Asset: allows you to save the asset as a file, which can later be imported in Assets Panel.
Save As Asset: allows you to directly save to Assets Panel without saving to a file.
Choose Export Asset option if you want to use the asset in another instance of Animate, or if you want to share the asset with others.
Please note the asset added will be shown under Animated or Static section of Custom tab of Assets panel
For more details please refer https://helpx.adobe.com/animate/using/work-with-assets-panel-animate.html
Thanks!
Mohan
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