Current configuration for single mailbox

Get-MailboxJunkEMailConfiguration -Identity "Your Mailbox" | Format-List

Junk Mail details only

(Get-MailboxJunkEMailConfiguration -Identity "Your Mailbox").BlockedSendersAndDomains

Add blocked senders

Set-MailboxJunkEMailConfiguration –Identity "Your Mailbox" –BlockedSendersAndDomains @{Add="JunkMailEmailOrDomain@spam.online"}