I cannot seem to figure out how to create a new record “under” a specific connection. For instance, suppose I have a table of clients and a table of invoices. I connect the invoices to the client as usual. Whenever I want to create a new record under the invoices table, it makes me choose a client from a drop down menu. That’s fine if I have two clients. But suppose I have 1000 clients. I want to be able to choose the client first, then create the invoice record under that specific client. Is that functionality not possible?
Welcome to the Knack Community, @havemaj!
One way to accomplish this would be to have a Details page with a Detail’s view to a specific Client record. Then, you can include a form view to insert a new Invoice record connected to the page’s Client:
Please let our support team know if you need further assistance with your setup or have any questions about anything else. You can submit a support ticket via this form. They will be happy to help! 
Happy new year! 
You can do it that way. If you have 1000 clients you would just begin typing in the name and Knack use the standard progressive lookup that you might expect.
But more typically, from your Client grid add a link to a new detail page. Add the Invoices grid to that page. And add an Invoice form to add the Invoice for that particular client.
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Absolutely Fantastic!! That worked!!
That worked fantastically. May I ask another question: Can this system do math? For instance consider my invoices from my example. Each has a dollar amount attached. Suppose I add additional records with client payments. Can this system perform a running tally and display the result in a field?
Glad to hear that worked out for you!
Yes, equations can be used to calculate a total tally and display the result in a field. See our article here on Equations to learn more about using this field type.