In case you want to tag a connection you just need to find that specific connection and then click on the ellipsis (⋯) - More Menu - on the right-hand side of the connection row and select Apply Tag option.
You can also perform the bulk Apply tag by ticking the checkbox on the left-hand side for the connections you wish to apply the tag to and clicking on the Apply Tag button.
Once you click on the Apply Tag option/button, a popup will appear presenting the field where you can:
Once the tag is selected/created, the success toast notification will appear confirming the successful action.
The applied tag will appear in the Connection list in the Tag column.