Organizing exhibitions falls under HSN 998596 and it attracts GST @18% (9+9). In terms of Section 13(5) of IGST Act, the place of supply is where the event is held. Since the exhibition is held outside India, so it is export of service subject to fulfillment of other conditions.
Secondly, it appears to me you are playing the role of intermediary services. See the defintion of 'intermediary' as defined under Section 2 (13) of IGST Actwhich is as follows:
(13) “intermediary” means a broker, an agent or any other person, by whatever name called, who arranges or facilitates the supply of goods or services or both, or securities, between two or more persons, but does not include a person who supplies such goods or services or both or securities on his own account;
If your activity/supply falls under the category of 'intermediary' the place of supply will be in India as per Section 13(8)(b) of IGST Act. Tax rate remains the same.
How consideration is accounted for ? That is also crucial to arrive at final decision for correct classification as well as payment of tax.