How can I integrate this plugin with a third-party login form?

Several third-party login forms are already integrated; for the up-to-date list, see the product page.
Some not named there will also “just work”, because they have called the same internal login hooks in WordPress as supported login forms.
On the vast majority of login forms (unless the form’s code is very unusual), the user can append their OTP code (i.e. the two-factor code) to their password. In this case, you should just add some text above your login form to tell users with two-factor turned on to do so.
Concerning developing new custom code for you in general, or reviewing your custom code in general, please see the FAQ “Can you write some code for me, to customise the plugin?”.
A competent PHP developer will be able to read the code in the plugin in the file simba-tfa/includes/login-form-integrations.php and see the WordPress hooks to use in order to integrate with a new login form.
Usually, if you can provide us with a test WP site (dashboard and FTP or SFTP access) with a suitable login form already set up, we can develop a new integration for inclusion in the plugin for a small fee; please contact our support to ask. Very occasionally a login form is coded in an unusual way that makes any integration impossible; in this case you would be refunded.