BASIC Auth Jersey REST API

Question

I am trying to secure my REST API with BASIC authentication. I am doing this by defining roles within my web.xml and tomcat-users.xml like so:

web.xml

<servlet-mapping>
    <servlet-name>Jersey REST Service</servlet-name>
    <url-pattern>/rest/*</url-pattern>
  </servlet-mapping>

  <security-constraint>
    <display-name>Secure REST Area</display-name>
    <web-resource-collection>
      <web-resource-name>Secure REST</web-resource-name>
      <url-pattern>/rest/api/</url-pattern>
      <http-method>POST</http-method>
    </web-resource-collection>

    <auth-constraint>
      <role-name>admin</role-name>
    </auth-constraint>
  </security-constraint>

  <security-role>
    <role-name>admin</role-name>
  </security-role>

  <login-config>
    <auth-method>BASIC</auth-method>
    <realm-name>UserDatabaseRealm</realm-name>
  </login-config>

tomcat-users.xml

<role rolename="admin"/> 
<role rolename="user"/> 
<user username="admin" password="admin" roles="manager,admin,manager-gui"/>
<user username="user" password="user" roles="user"/>

The POST request that I am securing is annotated as follows:

@POST
@Path("/validate")
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
@RolesAllowed("admin")
public Response postRequest(Data inputData) throws FileIOException {
.
.
.
}

The request is secured, and I get a HTTP Status 401 – Unauthorized when I attempt to hit it, but after adding BASIC auth header with the 'admin' 'admin' username/password, I am still seeing the unauthorised message.

In Postman:

Is the role not configured correctly? Or am I attempting to add the Basic Auth header incorrectly?

Thanks for any help.