You can use the following curl command:
curl -v -u firstname.lastname@example.org:test -X GET 'https://domain.freshservice.com/api/v2/ticket_fields'
If in case you do not want to use your email address and your password in the curl command, you can also use your API key instead of the email address and X as the password as shown below
curl -v -u API key:X -X GET 'https://domain.freshservice.com/api/v2/ticket_fields'
For more information, please visit https://api.freshservice.com/v2/#get_all_ticket_fields