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I have just installed ScriptRunner for Jira Cloud and I cannot find the same option to create Script Fields that I have inside of ScriptRunner for Jira Server.
The ScriptRunner for JIRA Cloud plugin is unable to provide the same Script Fields functionality that the server version is able to provide.
The reason we are unable to provide this functionality is due to the restricted functionality and API's which Atlassian provide inside JIRA Cloud, due to the fact that Atlassian only provides a Rest API in Jira Cloud and not the same Java API that the server version contains.
You can see more detailed information on the differences between ScriptRunner for JIRA Cloud and ScriptRunner for JIRA Server inside of the documentation page located here.
However, we can confirm that the closest functionality to a Script Field that can be achieved in Jira cloud is a Script Listener fired on the Issue Updated event. which calculates a value on an issue as described inside of the documentation page here.