Hello!

I need to create a custom transition screen. It doesn't matter in what way (I'm ready to write an additional plugin), you need to display the window (as in the picture) during the transition. The issue in which this window will be called has "custofield_11300". There are no "ParamN" fields in the issue. When you click on the "Calculate" button, the sum of the three fields should be calculated and the result should be written to "custofield_11300" for the issue for which the status transition was called.
I use jira server.
I have never created this, please tell me how this can be done?
It is not necessary (but desirable) to call the modal window on the transition, you can make a separate button for calling.
If there are no solutions, advise an alternative. (Maybe a separate servlet?)
Thanks!