Button Application Link where source field is empty

For an association where I have a member table and a guest list table I have the following Issue:
The guestlist has members and non-members.
From the single form of my guest list, I need to be able to open the member single form showing the linked member info, if it is not a member (Null) or non-existing member (wrong number), it should open the member form in insert mode, so I can add a new member.
When I use the Application Link in Scriptcase for a button on the Guest List Single form, it works like a charm if I have an existing member number, or when I have a non-existing member number (open form in insert mode), but when the member number on the guestlist is empty, it opens the form with the first record from my member table.
Would be great if somebody have a solution to this issue.
Thank you in advance.