Sending emails to custom multiselect list of users by workflow when there is any change in opportunity


I have added a custom multiselect field with suiteCRM users (type: multienum) to opportunity - it works.
I have WorkFlow Module: Notes with Select Action : Send email to Related Field “Users: Assigned To”. So if any change is made/there is added some notes to history of that opportunity, suiteCRM sends email to this person.
But instead of that I want to send email for my custom multiselect list of users chosen for selected opportunity.

Does anyone know how to use custom field with type of multienum like a related type like “assigned to” or “created by” in workflow module? Is it possible to make it by studio or only inside the code? Which files I need to edit?

I will be grateful for any help!