Getting a 403 error when sending odata update request to S/4HANA public cloud

i am building an app that updates data in S/4HANA public cloud via an app. It is using a standard OData update record. I have created a flow with ‘update record’ logic after pressing a button. When testing this (or testing the OData directly in the ‘Data’ part, it gives a 403 error. Does someone know how to solve this kind of error? Could it have something to do with tokens? In that case i am wondering how we can add the token to the update message.
many thanks,