The Linked Issues Field
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:
getFieldById("issuelinks-linktype").setFormValue("is blocked by")
getFieldById("issuelinks-issues").setFormValue(["SSPA-1", "SSPA-2"])
Which will result in:

Other operations
If you issuelinks
as the identifier it will make both the direction and the issues readonly, required, etc, eg:
getFieldById('issuelinks').setReadOnly(true)
Alternatively you can operate on either of the individual fields:
getFieldById('issuelinks-linktype').setReadOnly(true) getFieldById('issuelinks-issues').setReadOnly(true)
For how-to questions please ask on Atlassian Answers where there is a very active community. Adaptavist staff are also likely to respond there.
Ask a question about ScriptRunner for JIRA, for for Bitbucket Server, or for Confluence.