When the custom questionnaire was created, did you remember to give it a name and save it?  If so, it should show up as an option from the dropdown.  Otherwise, you may need Intuit support to look into it for you on the back-end.

Here're two articles about the process.  Although these are slightly dated, they are still good references, nonetheless.

https://http-download.intuit.com/http.intuit/CMO/accountants/intuit-link/questionnaire-templates.pdf

https://accountants-community.intuit.com/articles/1861087-using-intuit-link-in-lacerte