To view marketing interactions, we need to add a custom connector to the timeline of the lead and the contact form. This step is only required if we are using custom forms and not the out-the-box marketing forms for lead and contact.
The OOB Form already has this connector defined.
Open the form for customization, select the timeline control, scroll down to Advanced >> Additional Settings >> Custom connectors, and click on Add connector.
Specify Constructor and Resource Path and save the record.
Resource path: msdynmkt_DynamicsMktTimelineInteractionAnalytics
Save the form and publish it.
We can see the marketing interactions being displayed in the timeline.
Hope it helps..