You can find out the main FTP user login details from the psa database of the Plesk. Using the following queries you can obtain the main FTP login details of the domain.

This following queries are used for Linux Plesk.
mysql -uadmin -p`cat /etc/psa/.psa.shadow`
mysql> use psa
mysql> select id from domains where name='EnterTheDomainName';
select * from hosting where dom_id='1'\G;

Note: You need to add an extra field for port '-P8306' along with mysql command to connect to the windows mysql. Also you need to exactly specify the mysql password instead of cat /etc/psa/.psa.shadow