Update formula field from custom action

Update formula field from custom action

I have a form that contains (2) fields called Reg_Users (lookup multi-select) and Reg_User_List (formula). The reason for the formula is due to the View criteria not working correctly. Example: If you were to set the criteria as Reg_Users == zoho.loginuser it will not work correctly, but if you use Reg_User_List (Formula: "" + Reg_Users) it works.

My issue is the formula field only updating if you edit/update the form. The application uses custom actions to update the Reg_Users field, which does not update the formula field. What script could I use to update the formula field?