Unfortunately, JanusGraph does not support basic authentication on its SOLR client yet, see https://github.com/JanusGraph/janusgraph/issues/1056

Before finding this JanusGraph issue, I came across the following piece of SOLR documentation which might be worth a try:

https://solr.apache.org/guide/8_1/basic-authentication-plugin.html#global-jvm-basic-auth-credentials

Finally, note that the discussion in the JanusGraph issue also includes a workaround forapplying  ssl on the SOLR connection, which is a natural further step for protecting your SOLR data.

Best wishes,     Marc