How do I create a custom partial / template?

The thing you’re searching for is get_template_part function.

So let’s say you put your form in file called part-form-signup.php. Then you can easily include that partial template anywhere using:

get_template_part( 'part-form-signup' );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)