SharePoint Framework

You don't need SPFx, this can be done with a Power Apps canvas app - note: not a Power Apps customised SharePoint form. Your 2 lists would be the data source. On the screen you'd have a gallery listing all the sales leads. When you select one it displays (in a separate container) a form for adding a new interaction. You could then have another gallery listing all the interactions for the selected lead and be able to open if one if you needed to edit it. Something like that anyway. 

Edit: I spent an hour (well, it is a Sunday morning so I'm not working and my wife is out) building a very quick & dirty system. The 2 lists:

This is the main screen of the app. The user selects a lead from the gallery on the left. A new interaction form is available for them with the details from the Sales Leads list.

If the user selects a different lead then that information is displayed on the new interaction form and the interactions for that lead are displayed in the table below.

Hope that helps, come back with any questions.

Rob
Los Gallardos
Microsoft Power Automate Community Super User.
Principal Consultant, Power Platform, WSP Global (and classic 1967 Morris Traveller driver)