Your requirement is to query Marketing Leads based on some filters using GET function in Postman / SoapUI. You want to know what is the Entity name which can be used to query the Marketing Leads.
Sales Leads has gone Obsolete in SAP Cloud for Customer system. For Sales Leads in Sales work center, the Entity is Lead.
There is a new Entity for Marketing Leads which is LeanLead. For Leads in Marketing work center, refer to entity set LeanLeadCollection.
KBA , LOD-LE-MKT , Marketing , How To