From Longman Dictionary of Contemporary Englishwrite away for something phrasal verbASK FOR something/ASK somebody TO DO somethingto write a letter to a company or organization asking them to send you goods or information I’ve written away for their free catalog. → write→ See Verb table
Examples from the Corpus
write away for • He scanned the list for obscure schools in certain locations, then wrote away for an application.• Rose helped Maggie to write away for the forms and then to fill in the forms when they came. 