You can find an answer to your question in the following article:
The idea is:
To Set epic link you have to use the same api as to set any other custom field value to an issue. First you get reference to the Epic link field. Secondly you set it using MutableIssue or IssueService like in the example above.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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