Torah Temimah cites Shevuos 35a.
If he sent via his slave, or the defendant said to them “I adjure you that if you know testimony for him you shall come and testify for him” — they are exempt, until they hear from the mouth of the claimant.
The adjuration must come from the claimant himself. Transmission through a slave or an adjuration issued by the defendant does not create shevu’as ha-eidus liability.