We are trying to create a campaign and for that campaign when we try to add a flow.
In the flow, we have added “Add Data” for object “activity” as showed in the screenshot
When we click on the column values to set option, a list of fields are displayed but unfortunately “Contact” field is not available.
As activity is OOTB created object, by default it does contain contact and account fields but we are not sure on why it is not being listed in the dialog box
Any object created by the campaign (e.g. activity) is automatically connected to the contact entity via business logic of the campaign. There's no need to connect it manually.