Find a customer's accountID

[FrancoB]

Hello everyone,
I wanted to ask for confirmation about a problem I'm having:
Through automation, I update a custom Email field by entering an email address that is generally already present in the project's Customers (portal access) or is a Jira user.
Now, in the automation, I would also like to be able to enter request participants, and since there may already be some present, I thought I would do an update via JSON using the account ID, as when I try to use “Copy from Work Items” and add the email previously written in a variable, it sometimes overwrites the request participants already present.

So I thought I would retrieve the accountID via web request, but it doesn't work.

E.g.:
rest/api/3/user/search?query={{issue.customfield_156.urlEncode}}

If I try to run the string directly in the browser, entering the email address works and displays it.
E.g.
rest/api/3/user/search?query=email@email.com

How can I do this? Any advice on how to handle this case?
Thank you