CRM Contacts and Zoho Creator

CRM Contacts and Zoho Creator

I would like to link one of my forms with a contact in CRM. I've made an Integration field but it let me show only one field, so I've choose Last Name. Unfortunatelly it's not enough to identify a contact.
I also tried to make a custom field in CRM with a formula containing both First and Last Name, but it seems Zoho Creator doesn't let me to use custom fields on Integration fields.

Any other idea will be welcome, thanks in advance!