Can I import data to CRM field?

Can I import data to CRM field?

I have a form in my application into which I am wanting to import data from a CSV. 

The form contains a CRM field. 

I have tried importing using the Lead_ID from CRM as the value for this field, however this does not work. 
I have tried importing using the Lead_Name from CRM as the value for this field, however this does not work either. 

Can anybody tell me what value can I use on import to link to the correct record in CRM?

Cheers, Bianca.