Custom UI, modal window on to of create issue screen

[Viktor Fedun]

Hi, I can create issue as normal and write for example users credentials in custom_fields there.
But I do have to implement ability to search/filter user by name, surname, birth, mobile, etc. To do that I decided to create custom UI that should be shown on top of create Issue screen and should autofill all customFields after serch is done and modal closed.
what way is the best to implement this?

I tried adding .setHelpText(<div onClick="setCustomFieldValueSomehow()">myHtmlHere</div>) into scriptRunner behaviour but seems that's pretty hard to add complicated html this way. Also I wasn't able to find a way to set customField value by JavaScript on page
is there any better way of creating button that is able to set customFields values?