I'm trying to create a form that adds data to a Marketing Events SharePoint list. The form has a number of fields, most important of which is a multi-select field that pulls from another SharePoint list, Contacts. The Contacts list is very large, in the thousands. I need to build the form so that the user can choose options (either checkboxes or dropdowns) that filters what the lookup field pulls from Contacts based on the options the user chooses.
I currently have a multi-select field that is configured as a lookup to pull from Contacts. However, the list it pulls is obviously unfiltered. An example of a filter I'd like is a checkbox named 'MAO' that if checked, would make it so the multi-select field would only show entries from contacts where the value of the office column in contacts equals 'MAO'. Or a dropdown box that has a list of accounts and the list is filtered by that.
This is SharePoint Online. I have access to SharePoint Designer 2013, InfoPath 2013, and i've installed SPServices, though the functions in that don't quite match what i'm trying to do. Any help would be appreciated. Thanks