Site / tool to contact multiple study participants with their IDs after pseudonymization

For this multiple-weeks research study with an early access app, a pseudonymization process is used to disconnect the participant's emails from their data with a random ID.

I need to contact those people after they registered through the pre-form and provide them with the app link and their ID. They will need it to fill out the post-form and provide the ID so the data of this second form can be connected to the data of the first.

Now the thing is that I have about 60 people and it would be annoying to send each of them an individual mail with their ID.
Can anyone recommend something that would make this easier?
To somehow automate the process of sending out the correct ID to the correct email?

---

I've thought about tools like Mailchimp, but you can only dynamically reference variables that the user provided (e.g. name or gender).