plesk not accepting my remote ftp login :/

[EvolutionCrazy]

I've a server that i want to use to place backups using "FTP account for remote repository"

the login on the remote server is structured like:

[email protected]

when i try to use that login on the plesk panel, plesk block me and say

"You can use lowercase alphanumeric, dash and underscore symbols in the login name.
The login name should begin with an alphabetical simbol.
The password should not contain the login name"...

now i'm asking: why this??? how can plesk block me configuring a remote ftp???

how can i evade this limit in the "Remote ftp repository"?
I cannot change my username on the backup server...