At the sense level, you can insert a lexical relation (such as Antonym) and then add a reference to it.
For the entry level, see Insert a cross reference lexical relation.
In the Navigation Pane, click Lexicon, and then click Lexicon Edit.
In the Entries pane, click the desired entry.
In the Entry pane, if you cannot see the Lexical Relations field in the desired sense or subsense, show hidden fields.
Click the Lexical Relations field label to display the menu button 
or right-click the label.
Then click one of the following:
Insert <name> Relation — this inserts that particular lexical relation.
The Add Reference (<name>) dialog box appears. See Add reference to lexical relation for help with the dialog box.
Create New Lexical Relation — this opens the Lexical Relations tool (in Lists) and inserts a new lexical relation there.
Select a reference set type for the new lexical relation, and then add information in the fields to define the relation.
Click 
on the toolbar to return to Lexicon Edit.
The new lexical relation is available for you to insert, as described above.
Lexicon Edit overview (tasks)