How can I get the ticket fields using APIs?

You can use the following curl command: 

curl -v -u user@yourcompany.com: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