Usually, a validated scale is published in a peer-reviewed article. You need to search for the article on Google Scholar using the title of the scale. Once you find the article, you will see the name of the corresponding author and their email address. You can then email them and seek permission to use the scale.
If the full content of the scale has been published in a peer-reviewed article, then you typically do not need additional permission to use it. Otherwise, proceed as described s. Rama Gokula Krishnan
cite or write as adopted or adapted from .........(name of the author and the year of publication, if you want to add more, give full description what you have borrowed.
I agree with the previous recommendations. Nonetheless, I strongly recommend seeking permission from the first author to use the scale. It shows appreciation and can even help build a network that can provide insights to further develop the research.