How to define an index, within a Sequelize model?

It can work in single migration also.

In my case, just perform the addIndex after createTable method in the migration file

Migration:

return queryInterface.createTable('Item', {
    // columns...
}).then(() => queryInterface.addIndex('Item', ['OwnerId']))
.then(() => {
    // perform further operations if needed
});

it's work for me in the migration file.

Almost there. You should add indexes in a new object like this:

module.exports = (sequelize, DataTypes) => {
    const Item = sequelize.define('Item', {
        itemId:  DataTypes.STRING,
        ownerId:  DataTypes.INTEGER,
        status: DataTypes.STRING,
        type: DataTypes.STRING,
        nature: DataTypes.STRING,
        content: DataTypes.STRING,
        moment: DataTypes.BIGINT
    },
    {
      indexes:[
       {
         unique: false,
         fields:['ownerId']
       }
      ]
    });

    return Item;
};