In fact, there is two possibilities to add phpMyAdmin to Laragon.
First :
Like previous dev said before, it is possible to take the last zip on phpmyadmin website, unzip it and put the extracted folder in laragon/etc/apps. Take care to rename your unzip folder with this exact name : "phpMyAdmin"
Second :
You can simply use Laragon QuickAdd functionnality.
But by default it installs the 4.X version of phpMyAdmin.
Hopefully, you can configure wich version of phpMyAdmin QuickAdd will use.
To do that go to Menu > Tools > QuickAdd > Configuration... and under the line :
# phpMyAdmin
*phpmyadmin=https://files.phpmyadmin.net/phpMyAdmin/4.8.5/phpMyAdmin-4.8.5-english.zip
You can add the last link available for phpMyAdmin (you can find it here) :
# phpMyAdmin
*phpmyadmin=https://files.phpmyadmin.net/phpMyAdmin/4.8.5/phpMyAdmin-4.8.5-english.zip
phpmyadmin5=https://files.phpmyadmin.net/phpMyAdmin/5.0.4/phpMyAdmin-5.0.4-all-languages.zip
And now when you'll go in QuickAdd, you will see that.
So you can now install it. When it's done, Laragon open up automatically your default browser with phpmyadmin at this url http://localhost/phpmyadmin
For the french community, I add a video on this subject.