Convert the SQL Server script to MariaDB script

Question

I want to convert the SQL script from SQL Server to MariaDB of the following below but I can't figure out.

IF NOT EXISTS (SELECT * FROM sys.database_principals WHERE name = N'client')

CREATE USER `client` FOR LOGIN `client` WITH DEFAULT_SCHEMA=`dbo`;'

ALTER ROLE `db_client` ADD MEMBER `client`

ALTER AUTHORIZATION
    ON [ <class_type>:: ] entity_name
    TO { principal_name | SCHEMA OWNER }
    [;]

What is the best way to migrate SQL Server to MariaDB?

Or any equivalent of MariaDB?