I'm sorry - I'm not really clear on what you're asking here. But before creating a form with SeaGreen, you have to give your database a name, a username and a password. SeaGreen takes care of the rest.
The most common problems encountered with this are:
1. The database information you entered into SeaGreen is incorrect. Most commonly people enter the wrong database host name (not always "localhost") or username/password combination.
2. The user you create for use with the database may not have the proper permissions to write to the database. Your ISP should have instructions on how to properly establish read/write permissions for your database user.
With regard to your second question - when you merge a SeaGreen form with an HTML file, your form is now a PHP file and can be found with the name that you gave that form in the SeaGreen publish step.
So if you create a stand-alone form called "contact" - it will be found on your website by the name "contact.php." If you merge that form with a file named "Contact_Us.htm" your form will still be called "contact.php" because that's what you named it in SeaGreen. At no time will a SeaGreen form have an .htm or .html extension. It will always be a PHP file.
Once you've merged your form with your HTML file, you still need to publish that form via the SeaGreen publish interface.
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