I do not know that the Kibana dev tools supports passing the username and password like this. I think you don't need to include this, because you already logged into Kibana.
You are confusing the syntax for curl with the syntax that is permitted in Kibana. You just write
PUT /_snapshot/my_backup/snapshot_20181011
{
"maybe": {"some": "more"},
"options": "here"
}
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