Use ContactID to Identify Contacts

I am a ConstantContact customer and was advised to reach out to you via Support:

I need help with the following add-on that exists in ConstantContact to connect to SurveyMonkey - it is supported by ConstantContact so SurveyMonkey is unable to help me: https://www.surveymonkey.com/apps/sMG9kYyRHI1pBSkoRChs_2FA_3D_3D/details/ 

 

I connected a ConstantContact email campaign and included a SurveyMonkey block, but am trying to identify users who partially responded to the survey and did not submit (btw ConstantContact also doesn't offer much in terms of what responses the users provided). 

 

Talking to SurveyMonkey, in my SurveyMonkey survey response, I have a custom data field that looks like this:
Custom Data: {"ev":"1","contactId":"eEEYgvDyrZQYolAPJMiidVugUMThbSixpXhTDETOVrqCS1V5FPHwSQ==","campaignActivityId":"UeUeG0wF00HZEIRN-ojt5j5VlGIiOT8vsPxkM1WGYReducbG1BqaOw"}  

I need to know which Contact does this contactId belong to: contactId":"eEEYgvDyrZQYolAPJMiidVugUMThbSixpXhTDETOVrqCS1V5FPHwSQ==" 

 

 I tried using APITester with ConstantContact APIs but got this error:
"error_key": "http.path.param.id.invalid",
"error_message": "Invalid ID. Acceptable formats are positive longs or UUIDs" 

 

  I am not sure how to query ConstantContact with that contactID  - how do I convert what ConstantContact is passing on to SurveyMonkey for ContactID back to a UUID or Long, so that I can then call the contacts/contactID API to get the Contact that responded to the survey?