Currently tagging specific changesets is supported only in the command line mode in the embedded Terminal. To launch the Terminal, hover your mouse pointer over in the lower left corner of the IDE, then choose Terminal from the menu (see Working with Embedded Local Terminal for details). After commit, tags appear in the Log tab of the Version Control tool window.
For more information about Mercurial tags, see Mercurial Documentation: Tag.
Tagging a repository
IntelliJ IDEA provides UI for tagging the current repository which means assigning a tag to its
tip. The created tag is global, is stored in the file
.hgtags. After commit, the tag appears in the Log tab of the
Version Control tool window.