I have 18 checkboxes on my document that I want to list in a dropdown where multiple values can be selected.
Is there an API listitems attribute to make the dropdown a multiselect? I have looked but could not find it in API. It looks like it is available in Connect but we are not using that.
Unfortunately this is not possible with the eSignature REST API. Dropdown tabs can only allow the user to select a single value and if you want the user to select multiple values you would need to use checkboxes like you indicated in the question.
According to this article, it looks like this is an option with document generation in DocuSign CLM, but not through the eSignature REST API.