Can you clarify what you're exactly trying to do?
Are you using a Zapier Interface to generate the form and then you've just linked it in your navbar and you want to make the link dynamic to include the member_id ?
That's probably a bad approach, and if there are critical data or if you have to guarantee that the member who's submitting the form is really the member who's ID was submitted, you'll need a better approach, because literally anybody could open the URL and just adjust the variable.
That said, back to your question:)
You'll either need a custom application with a custom navbar extension to build the dynamic link, or you could use JavaScript to customize the target link and attach the currently logged in member_id