You will not be able to set a custom send ID using the API, but you can request Sinch to configure a custom sender ID for you. Contact your Sinch Account Manager who will be able to help you set this up.
However, to provide the highest deliverability and verification success rates, Sinch recommend using the already existing optimised sender IDs, which are pre-registered for each market. This ensures that your verification SMS will not be confused as marketing or spam with carriers across the globe and reach your users without fail. You can find additional setup information on Sinch Developer Documentation and learn how to get started with SMS verification.
Click here to learn how you can use Sinch SMS Verification to protect your business and your customers.