Dear @kunti pawar ,
I assume that you would like to have the same functionality the "Comment" has currently to "copy" to a custom field.
Unfortunately this is not possible. Comment is a system field. You could just add a custom field of type text-multi-line and name it eg. "Comment2", but it will not be treated as a "Comment".
If you would like to spend some money, you could have a look at the marketplace of Atlassian. Eventually there is a vendor, who offers extras for comments.
So long
Thomas
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