We are doing a joint campaign with a client who is paying us to get customers would it make sense to pay people money to get onboarded and use our platform how would u go ahead with such a marketing copy for landing page?

it's simple you can use facebook ads

We have a list of people we want to target

The main question that needs to be answered BEFORE you walk down the road further is are these people on the list willing to pay for your product. That is literally the only question that matters.

Also I would suggest that you narrow your target audience to include conversions for only the actions that the client wants. That way they are not dissatisfied with the engagement / quality of visitors that you send.

We also have a list of customers that need to be targeted, email list

If you have an email list, do cold email. If you want to run ads, upload the email list to FB and map them. Also, create a look like.