Code

use the custom js editor to this. see this tutorial for an example

hii,,

I see there is a js addition function in the video, can you tell me how to add the addition function?

like text01 + text02 = text03

or some sum like
text01 + text02 + text03 + text04 = text05

not sure what you mean by addition. However, if you are referring to string concatenation, then there are plenty of available resources that you can find with a google search.

adding a calculator function to the form,

like the sum of field1 + field2 = field3

You’ll have to do this with custom code. You can see this tutorial for help.