Get Sprint Name for Issue

// The issue key
final issueKey = 'TEST-1'

// Fetch the issue object from the key
def issue = get("/rest/agile/1.0/issue/${issueKey}")
    .header('Content-Type', 'application/json')
    .asObject(Map)
    .body

// Get all the fields from the issue as a Map
def fields = issue.fields as Map

// Get the Custom field to get the option value from
def sprintName = fields.sprint.name // Note change .name to .id to get the ID of the sprint.

"The name of the current Sprint is '${sprintName}'"