You need to call the CreateOnboardingLink API endpoint and pass in an AuthToken for that user as well as your application reference and a callback URL to receive the progress of the application and results.
You will need a user / authtoken for each type of unique 'journey' you need your users to perform, one may be sufficient.
To request AuthTokens and setup user journeys, please liaise with your TrustID Account Manager.
Here are 2 sample ones if you want to try it out
1. Standard Journey Type N5LA16a7PZZxBIsyMFH8YYYZQWo3/V4VJZN3jlClin0=
2. Right To Work Journey Type tBNWnrkHQibwf4gU2HBmqRB5VQ0b/EN9BH+B8TWWmeI=
Your application may need to be sent for manual review, we will call you back on the supplied URL when it has completed processing which may be just seconds but maybe later if it needs to be sent for review as it cannot be auto-validated.