You need to use check box or drop down. 

I normal use both when I do integrations. 

​@Daniel Söderlund  got there first :D 

I would also include a 3rd party reference field too 

Ofc,  custom ticket ID field is mandatory. 

What I do with the drop down field is to define the partner and trigger the workflow. 

The checkbox is used to check that there are a initiated integration with the ticket and the external partner. That way you can use it for business rules for forms and in workflows.