LinkedIn Lead Gen Forms Custom Checkboxes

In order to comply with GDPR we need an email opt-in checkbox on our LinkedIn Lead Gen Forms. Currently, Marketo is the only major marketing automation platform without this ability - Lead Gen Form Third-Party Integration – Troubleshooting | Marketing Solutions Help 

You can create a custom question but that places it at the top of the form and would have a default. Not a great experience. We want to be able to have in platform forms but still comply with GDPR email opt-in best practices. 

This works fine with Facebook Lead Ad forms. The only current solution is Zapier which introduces an unhealthy amount of work and maintenance.