In short, JIRA does not let you restrict the options of their fields.
ScriptRunner has the FormField.setFieldOptions() method, but that only works for select lists, radio buttons, and checkboxes.
The only workaround I can think of is creating a multi-select select list, fetching all the projects you want, then populating the options with those projects. With that said, this is a fairly involved process in which I have no experience, so I cannot provide any help.
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