Custom field value - finding internalId

We need to add a currency field to the oppotunity (to represent an amount already paid) in NetCRM, and we need to be able to set the value of this field through web services.

I assume we can use code like:
// Create a string custom field of type StringCustomFieldRef
SringCustomFieldRef strCF = new StringCustomFieldRef();
strCF.internalId = cfNSKey;
strCF.value = cfValue; //suitable for currency field?

It is assumed that the user has already followed the instructions to add a custom field to Opportunity, with a specified label, say "PaidField". In the example, it shows setting the internalID - I assume this is the identifier that appears in the URL - e.g. CUSTBODY1, CUSTBODY2, CUSTBODY3,....