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Unfortunately we currently don't have a possibility to run SIL scripts, so we have to use JIRA expressions instead.
When we you get to the workflow transitions we will you will be able to add conditions and validators using JIRA expression (for more details read Customizing workflows until the note linking here).
Now you can choose to add a condition using a Jira expression.
Then, you have the option to choose between a template or you can write your own custom script. For your convenience we have provided you with a wide range of templates "translated" from a SIL script to a Jira expression. This way you can either use the template you have at your disposition or you can inspire yourself from the already existing choices.
After you select a template the fields will autocomplete, then you can continue with the script as is, or you can customize it, rename it and continue.