For pre-filling the multiple choice field via URL parameters, the Field code can be found under the Advanced properties.
For selecting multiple choice, you can separate each of them with a comma.
The field code can be used in URL parameters, as shown below.
https://example.neetoform.com/8abc2aa31ef7d12f288e?proficient_languages=Ruby,C,Python
For using the other option, which provides a custom text input, you can add the other option to the URL parameters.
https://example.neetoform.com/8abc2aa31ef7d12f288e?proficient_languages=Ruby,C,Python&proficient_languages.other=Java