There are several ways to add a tag.
Only test specific is like:
*** Test cases ***
Test A
[tags] tagA tagB
Log This is test A
It is possible to add a tag to all testcases in your file by placing a Force Tags
in your settings:
*** Settings ***
Force Tags NewTag
For more information you can check the user guide: http://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#tagging-test-cases