You can't create portals to a "layout". You create portals to a table in your system.
You can set up a portal to submissions if you can define a relationship between your Leads table and your submissions table. A typical way to do this is to define a primary key field in Leads as an auto-entered serial number. Let's call it: LeadID. Then you define a matching LeadID number field (But NOT as an auto-entered serial number--just make it a number field.)
In manage | Database | Relationships, you can then drag from Leads::LeadID to Submissions::LeadID to link the two tables in a relationship. Double click the line this action created and click the "allow creation of records via this relationship" for Submissions.
Now you can go to the submissions tab on the Leads layout and use the portal tool to add a portal to submissions.