Invalid value for key 'attachdbfilename'

[esqueleto]

I have a 7.5.6 Plesk hosting with SQL Express installed.

I don't have access to the server, I only have an account in the server.

What is the connection string to access a database in the App_Data folder?
I try to use
data source=localhost\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|ExampleDB.mdf;User Instance=true

or

data source=localhost\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=C:\xxx\ExampleDB.mdf;User Instance=true

and I get always the same error?
What I'm doing wrong? What I have to ask the hosting provider?

tkx in advance
Paulo Aboim Pinto
Odivelas - Portugal