How do I associate an already created form with a Product?

First and foremost, setup the form. Please refer to this article <>

Once the form is activated, you will find "Form" as a separate menu on the left side menu bar.
Click the Form menu and the page loads up. On the page, you will see the "Create Form" button.
Provide name to the Form you  build, it will add to the URL. The URL still can be edited. Add description as per your form  and its functionality. Submit the details.
The right side panel comes up that has all the System define form fields.
Import them to build the form. Even if you don't find all the needed form fields in the System defined Form fields, no need to worry, you will have the option to add your own defined form fields.

If you wish to add more form fields, please select the system defined form fields and Click the Import Button. The Form builder will build the form based on the name you have provided.
You can see the page loads up and your Form name is displays. Click the Edit button and you find options to add Form fields.
Click and start adding form fields to the form that you have created.

Once the form is built. Please go to your Product dashboard where your product link or payment link is being created. Click on Manage button for the particular payment link to which you want to associate the form.

The right side panel comes up, you will see a  section named CUSTOM FORM and the dropdown "Select the Form". You will find all the forms you have created in your account. Pick the one you want to associate with this payment link. Click the  update button and you are done. The form is associated with the payment link.