Has anyone worked out how to get consent from a user within an app and then have that property reflected in Klaviyo so we can send service / marketing information to a user?

Hi I'm pretty new to Klaviyo - working for an app-based startup.

I think we will assume implied consent in the app (the act of downloading the app and creating account means they ae giving us permission to contact them with relevant information to help them run / use the service we are providing). How do I get that consent in the app to appear in the Klaviyo profile? Can it be done by API into the profile? Klaviyo forms does not seem relevant to our situation / service.