How to automat the addition of data in 1 link to another record's cell

HI people,
I work on a booking system for workout coach
They have differents mensual subcriptions plans for 4 to 8 sessions
I need to follow the number of session of each subscriber based on the plan they choose (up to 4 or 8)

So i have 1 table to follow the mensual subcriptions
And 1 table to follow the booking of the sessions.

I think to follow the number of sessions booking by each subscribers, i created a 3rd table based on the mails using for the booking.
For that I make a link to another record field of the booking session
and a rollup to count each time the mail appear (in the same cell).

When i add mail manualy in the link to another record cell it’s okay
But i want to automat this action but I still can’t find to do this
I tried Integromat and Airtable Script, that add data in the next line (cf: screenshot)

Can you help me ?

Regards,
Julien