You can work with the Linked Issues fields as with most other fields, except for a few cases. As it’s a composite field consisting of two fields, you must set the value differently from with other fields.
Setting the value
To set the value you need to set the two values separately:
Which will result in:
Reading the value
getFieldById("issuelinks-linktype") will return a String, i.e "blocks" or "is caused by".
getFieldById("issuelinks-issues") will always return an array, even an empty one.
issuelinks as the identifier it will make both the direction and the issues readonly, required, etc, eg:
Alternatively you can operate on either of the individual fields: