Using a external survey

Hello all,
I am new the qualtrics and I am liking a great deal about it but I have a problem. I have legacy surveys that cannot transition all at once. Looks like I could deploy an external link with panelist id appended. I cannot see a way of letting qualtircs know that it is complete.
Example would be
Surveygizmo survey. I have a link that i add ContactId to and post from Qulatrics. SurveyGizmo has webhook that i can send to let qulatrics know that the survey is completed for certain people. I do not know the format or where the information is to create this webhook. The purpose for this would so I can send reminders to those that have not completed.
If anyone has tried this it would be helpful to know or know where i can read about it.
Thanks for your time
Brendan