How to search person in Contacts and create a record in my own field? C# API

How to search person in Contacts and create a record in my own field? C# API

We have Contacts where we have a lot of people. 

So, I have the ID of someone and would like to create in the field (which I created before) a record for this contact.

I have created Responses and would like to create with C# API the record for this.


How I can do it? How can I get exactly this person in my Contacts? 

Thank you in advance!

Best regards,
Vladimir