You can use Admin endpoints to define the secondary currency for a site by setting the secondaryCurrency property.

To associate secondary currency to a site, issue a PUT request to the /ccadmin/v1/sites/{site_id} endpoint on the administration server. The following example identifies the secondary currency of the Site US as US Dollars:

PUT/ccadmin/v1/sites/siteUS

{
  "properties": [
    {
      "secondaryCurrency": "USD",
    }
}