To use FTP Attachments® with any of your custom objects first step is to create a lookup field in FTP Attachments® object which comes with the FTP Attachments® package.
On your Salesforce Account. Go To Setup -> Object Manager -> "FTP Attachments" Object.
Click "New" in the Custom Fields and Relationships section of the page
Choose the type of field as "Lookup Relationship" and click next
Create a lookup relationship field in the FTP Attachments® object for ’YourObject’ with the name say ’YourObject’, because it is a custom filed Salesforce automatically appends "_c" to it at the end. Choose the object for which you want to enable FTP Attachments® , in our case it is ‘YourObject’
Finish the creation of this custom field. Once done use the Custom Link Manage to create custom links for this custom object
After creating link button add it to the page layout of the custom object
Now, you are ready to use FTP Attachments® with that custom object.