To get click ID, you have to use API version two for your campaign and you would need to set "xxxclickidxxx" in your landing page URL which will be replaced the click ID value on affiliate traffic.

So, you have to set your CPA campaign landing page URL as something like this:

http://landingpage.com/page.htm?clickid=xxxclickidxxx

So, when an affiliate link is clicked, you will get a visitor to your page as:

http://landingpage.com/page.htm?clickid=XXX

where XXX would be the click ID.