As well as syncing a source system and adding all of its assets at once, you have the option to add a single asset into the catalogue.
Click on Add Asset at the top-right of the catalogue.
Provide a name for this asset.
Choose an asset type from the dropdown.
After you have chosen an asset type, provide the URL of the asset.
Click on Next at the bottom-right of the page.
If you have added attributes to this asset type, provide your values into the attribute fields here.
Add any tags for this asset using the Add button on the right.
Click on Save at the bottom-right of the page when you are ready and you can then view your new asset in the catalogue.