There's some more info about how to set up lead syncing with HubSpot in this Knowledge Base article, which covers how to set up syncing of existing lead ads, as well as setting up a consent field for your associated lead ads.
There's a bit of additional info about the consent field syncing in this product update that was posted last summer.
@talha I don't think that there is a standard boiler-plate consent. You should really run this past your legal team or a company lawyer. Making sure that they are familiar with the GDPR statutes, rights, limitations, etc.