Sending form data to Appointlet

We would like to prefill certain parameters on our Appointlet booking page from parameters sent when the user completes a form (with the Unbounce prefill form parameters script in place), goes to form step 2, then clicks a button to book an appointment on our Appointlet schedule. We have the Appointlet URL coded as the thank you page URL and append the URL parameters as usual. The issue is that although Unbounce does indeed send the URL parameters along nicely, the format required in Appointlet includes dashes in the field name. Specifically, we have set up these field names, and Appointlet does not allow underscores in them (the crux of the issue), while Unbounce does not allow dashes.

Unbounce field name / Appointlet field name:
field__first_name first-name
field__last_name last-name
field__phone_number phone-number

We are lucky with the email field which works with no special coding:
email email

Appointlet has to have the URL suffix in this form to be happy and prefill the form fields:
?field__first-name=Jim&field__last-name=Hill&email=email@email.com&phone_number=248-555-1212

If we could use dashes in the Unbounce field names there would not be any issue.

Has anyone created a script that could remedy this situation? We only need just those four values to be sent into Appointlet. I could POST it to Zapier and then email the lead a slick, customized booking URL, but I would sure rather have it just redirect to the Appointlet booking page and carry the required values.