This question is vague, but there's a couple of things you probably mean.
First, behaviours (which customize the issue creation and editing UI) aren't yet compatible with JIRA Service Desk. https://productsupport.adaptavist.com/browse/SRJIRA-728
Script Fragments, on the other hand, are compatible with JIRA service desk, and can allow you to modify the UI there. https://scriptrunner.adaptavist.com/latest/jira/CustomisingUI.html
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