how to set assignee in post function using script runner 'Modify issue'

[Ramesh Kale]

I want to set assignee 'abc@def.com' using post function script runner 'Modify issue'.

Default available script to set assignee is

issueInput.fields.assignee = [id:'123456:12345a67-bbb1-12c3-dd45-678ee99f99g0'] as Map

How is this script works ? what exactly this 'id' means? where i will get such ID for my users?

Someone please provide me the one liner script (with correct syntax) where i can just put the mail id of the user.

I am new to the Jira and not really good at scripting.

Please help me on this and if possible please elaborate your answer briefly as i have a deadline to submit this.