Your JSON array is wrong, instead of
"params":[{"email_part":"test86","domain":"domain.tld"}]
just use
"params":{"email_part":"test86","domain":"domain.tld"}
Difference (as php would view it):
[
"command" => "Emails.add",
"params" => [
[
"email_part" => "test86",
"domain" => "domain.tld"
]
]
]
and the correct syntax:
[
"command" => "Emails.add",
"params" => [
"email_part" => "test86",
"domain" => "domain.tld"
]
]