The general way to add anything from the palette is, click the element in the score you want to attach it to., then click the palette element. If you've got some extra time to kill, you can also use drag & drop, but you do have to be sure to drop it on a specific element in the score (wait for the drag icon to change to show you that you've reached a valid target).
Comments
See https://musescore.org/en/handbook/tempo. There may be some links to other parts of the manual that you may need to read as well.
The general way to add anything from the palette is, click the element in the score you want to attach it to., then click the palette element. If you've got some extra time to kill, you can also use drag & drop, but you do have to be sure to drop it on a specific element in the score (wait for the drag icon to change to show you that you've reached a valid target).