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:

https://example.filemail.com/[email protected]&message=test message&subject=Alternate subject&[email protected]

So the keys you can use are "from", "to", "subject" and "message".

For custom fields the formula is:

  1. Take the custom field label, remove all characters except letters and numbers - this gives you the query string key. 
  2. For textboxes: supply the desired text as the query string value.
  3. 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 (,)

Advanced example:
Let's assume you have custom fields setup as on this screenshot:

To populate "My custom TEXT 1":

  1. Query string key: "My custom TEXT 1" --> "MycustomTEXT1"
  2. Query string value: "My custom text"
  3. The final URL is https://example.filemail.com/?MyCUSTOMTEXT1=My custom text

To populate other fields:

  1. To check both checkboxes: query string key is "MycustomCHECKBOX2" and query string value is "1,2"
  2. To select 3rd radio button option: query string key is "MycustomRADIOBUTTON3" and query string value is "3"
  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:

https://example.filemail.com/?MyCUSTOMTEXT1=My custom text&MycustomCHECKBOX2=1,2&MycustomRADIOBUTTON3=3&MycustomDROPDOWN4=2

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