Is it possible to auto populate fields from a lookup field without hitting submit? 

I am working with two modules: leads & contacts

Leads module

I have a contact lookup field called 'Existing Client' and I have contact information fields.
If Existing Client is not null I would like it to auto-populate the client fields upon user input, since some of the fields are mandatory. 

I created a script (which I have not tested yet) to map the appropriate fields. 

2. // get record
3. lead = zoho.crm.getRecordById("Leads",leadid);
4. //lookup the value in 'Existing_Client' lookup field
5. clientLookup = lead.get("Existing_Client");
6. if(clientLookup != null)
7. {
8. // get contact record id
9. contactid = clientLookup.get("id");
10. // get contact record
11. contact = zoho.crm.getRecordById("Contacts",contactid);
12. // get contact information
13. firstname = contact.get("First_Name");
14. lastname = contact.get("Last_Name");
15. email = contact.get("Email");
16. phone = contact.get("Phone");
17. //create map
18. mp = Map();
19. mp.put("First_Name",firstname);
20. mp.put("Last_Name",lastname);
21. mp.put("Email",email);
22. mp.put("Phone",phone);
23. // auto populate lead fields
24. updateRec = zoho.crm.updateRecord("Leads",leadid,mp);
25. }

My problem? I cannot figure out how to populate on user input, only on submit/save.