Hi Winston,
Per my tests and research about Forms and Flows, I assume that your concern is about this Microsoft Flow template:
I also found that the suggested forms and SharePoint sites showing in the dropdown list is related the frequency of their usage.
Besides, to add a form that is not in the suggested list, you may try the following steps based on my test:
- Click Enter custom value in the bottom of the list;

- Copy the Form ID from the URL of the form:

- Paste the Form ID into the input box.
To add a SharePoint site that is not in the suggested list, the process is similar:
- Click Enter custom value;

- Scroll up back to the Site Address box and paste the site URL into it.
Best Regards,
Nathan