How to use SqlCommand to CREATE DATABASE with parameterized db name?

Parameters are supported for DML operations not DDL operations, there are no execution plans for DDL operations. you will need to use dynamic SQL

DDL = Data Definition Language (create, drop, alter....)

DML = Data Manipulation Language (select, update, delete, insert)

Sadly you can accomplish this by wrapping your DDL operation in a DML operation.

var createDatabaseQuery = "exec ('CREATE DATABASE ' + @databaseName)";

var sqlCommand = new SqlCommand(createDatabaseQuery, sqlConnection);
sqlCommand.Parameters.Add("@databaseName", SqlDbType.Text);
sqlCommand.Parameters["@databaseName"].Value = "HelloWorld";

sqlCommand.ExecuteNonQuery();