So the first place you’ll need to start is to figure out what success looks like to them (i.e. what is the return they want to see?)
It sounds like you’re going to essentially run a corporate event on their behalf, where they’re essentially the sole sponsor. If it’s a B2B client, then most likely they’re looking for leads or qualified leads, in which case to prove ROI you’d need to show you managed to attract x-number of prospective buyers/clients to the event for them.
You can infer this from their job titles and company names (assuming you capture this at sign up), or you could even use custom questions to delve further into their potential to be clients, by asking questions like ‘are you a budget holder / decision maker’ ‘what is your annual budget?’ etc.
You can then export these details and present it as a nice post-event report to your clients, saying that for their investment, they had (x-number) of qualified prospects in the room, accounting for a total budget spend of $Xm. If the cost per qualified lead at the event is lower than they usual spend, then you’re going to show them a positive ROI.
Of course that’s just one way to show ROI…other companies care more about things like brand lift (hard to show unless you have a very large audience and super buzzy social chatter going on around it), actual sales, engagement…there are a lot of objectives, so I’d just start there and work backwards.
Hope that helps (and actually answers your question!)