1. Create Quick Action to open the Screen Flow on the Parent object (Example - Opportunity)
2. Create List Button on the child object to reference that Quick Action.

Quick Action Developer Name: Add_Products_Related_List

List button syntax:
/lightning/action/quick/SOBJECT.QUICK_ACTION_DEV_NAME?objectApiName&context=RECORD_DETAIL&recordId={!CASESAFEID(OBJECT.Id)}&backgroundContext=%2Flightning%2Fr%2FOpportunity%2F{!CASESAFEID(OBJECT.Id)}%2Fview

Example:

/lightning/action/quick/Opportunity.Add_Products_Related_List?objectApiName&context=RECORD_DETAIL&recordId={!CASESAFEID(Opportunity.Id)}&backgroundContext=%2Flightning%2Fr%2FOpportunity%2F{!CASESAFEID(Opportunity.Id)}%2Fview