While browsing atlassian community I managed to find kind of interesting question – see below.
How do I automatically add watchers from a custom people field
https://community.atlassian.com/t5/Jira-Software-questions/How-do-I-automatically-add-watchers-from-a-custom-people-field/qaq-p/2432884
First thing to do would be using {{issue.supporters}} where “Supporters” is the Custom Field you set up for all the Supporters you want to watch the issue, but you cannot use this smart value.
You need to use smart value {{customfield_XXXXX.accountId}} where (customfield_XXXXX is your custom field “Supporters”) – The ID value (XXXXX) of your custom field can be found in the URL address while editing the custom field.