Usage patterns
〈事〉を口実にするto use something as an excuse/pretext
He used being busy as an excuse to skip the meeting.
口実を作るto make up an excuse
I made up an excuse to leave work early.
口実を与えるto give someone a pretext (for doing something)
That remark gave the opposition an excuse to act.
〈事〉を口実としてunder the pretext of something
The plan was postponed under the pretext of a lack of funds.