This script will set the labels fields to a certain value for all the issues that satisfy a defined JQL query.

import com.atlassian.jira.bc.issue.search.SearchService
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.web.bean.PagerFilter
import com.atlassian.jira.issue.label.LabelManager

// list of elements to consider for matching

def searchlist = ["foo", "bar", "bar"]

def labelName = 'Partner'

def labelManager = ComponentAccessor.getComponent(LabelManager)
def user = ComponentAccessor.jiraAuthenticationContext.getLoggedInUser()
def searchService = ComponentAccessor.getComponent(SearchService)

for (searchItem in searchlist){
    def jqlSearch = "company = $searchItem"
    def parseResult = searchService.parseQuery(user, jqlSearch)
    if (parseResult.isValid()) {
        def searchResult = searchService.search(user, parseResult.getQuery(), PagerFilter.getUnlimitedFilter())
        searchResult.issues.each{issue -> labelManager.addLabel(user, issue.id, labelName, false)
        }
    }
}