Programmatically share site with external users via passcode

Hello,

I have a large pool of persons external to my organization. Once a year they must complete a form that takes maybe 10-15 minutes to fill out. Each month I would like to send an email to some number of these people informing them that they must complete their form. I would like to do the following things programmatically (SPFx using React/Typescript):

1.  Use the user's email address to see if they are already in the "FormMembers" group.
2.  If not, add them
3.  Send them a notification email with a link to page that will itself will have a link to the edit form for the form they must complete (this is a form customization, again SPFx using React/Typescript)
4.  I want them to be able to access the site and form using a passcode that has an expiration date.
5.  I want to create and send the email programmatically
6.  I DO NOT want to require them to authenticate with anything other than the passcode. This is an annual event for them tha lasts only a few minutes.

Question: How do I programmatically create the link that I put in the email so that it will know to authenticate using a passcode?