Board Thread:Questions and Answers/@comment-30715799-20170205232304/@comment-24909831-20170206070826

Black Library Help page could help you, and if not, send an e-mail. But I do have a feeling that Black Library hasn't taken any fan submissions for years. Your best shot for publishing a story is by posting it somewhere in the internet. Here, if you like, dA, B&C... any I suppose.