You can generate a JWT in 2 ways:
 a Velocity ("Email Script") token
 calling a webhook and populating a lead field
 is the most streamlined once complete, but will require an advanced Velocity developer
 is easier to code, as the webhook service could be written in any language, but has a lot more overhead and will not be practical if you're sending > 15-20K emails per day, assuming your JWTs expire every day (thus you'd need to fetch a new JWT via webhook daily).