Use Ctrl-Enter do get a line break.
Please note that in the tag panel you have to set the comment field to "multiline" to be able to enter line breaks.
Right-click in the grey area of the tag panel and select "Adapt ..." in the menu or use File>Options>Tag-Panel.
There select the entry for Comment and click the Edit button.
In the drop-down list for "Size" select "multiline".