Sharepoint 2013 and Forms Based Authentication
https://social.technet.microsoft.com/wiki/contents/articles/23539.sharepoint-2013-form-based-authentication-fba.aspx (good article and verified as working, although Kerberos vs NTLM can be an issue, may need to try both)
https://www.c-sharpcorner.com/UploadFile/a9d961/forms-based-authentication-in-sharepoint-2013/