Sum up all the story points of stories under an epic as Total Story Points.
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional
Add the following expression:
sum(fieldValue({10106}, issuesUnderEpic(%{00015})))
{10106}
%{00015}
To achieve this, the following functions are used:
Once configured and added to an issue screen, the result will look like this: