When people go to your "Receive files" page they get an upload form, which is highly customizable. In addition to built-in fields (From, To, Message, Subject) you can specify any number of additional fields, which will be attached to the received package. This can be done here: https://www.filemail.com/companysettings#receivefiles.
It is also possible to populate all fields of that form dynamically using query string of the "Receive files" page. Example URL to populate built-in fields:
So the keys you can use are "from", "to", "subject" and "message".
For custom fields the formula is:
Take the custom field label, remove all characters except letters and numbers - this gives you the query string key.
For textboxes: supply the desired text as the query string value.
For checkboxes, drop down lists and radio buttons: as query string value specify index of the value that should be selected. If you want to select multiple checkboxes provide multiple indexes separated with a comma (,)
Let's assume you have custom fields setup as on this screenshot:
To populate "My custom TEXT 1":
Query string key: "My custom TEXT 1" --> "MycustomTEXT1"
Query string value: "My custom text"
The final URL is https://example.filemail.com/?MyCUSTOMTEXT1=My custom text
To populate other fields:
To check both checkboxes: query string key is "MycustomCHECKBOX2" and query string value is "1,2"
To select 3rd radio button option: query string key is "MycustomRADIOBUTTON3" and query string value is "3"
To select 2nd option in dropdown: query string key is "MycustomDROPDOWN4" and query string value is "2"
Full URL that populates those 4 custom fields: