Hi Swathi,
To join the marketplace you must join the Partner program, which any Wix user with four or more premium sites can do. Although one of the Marketplace services is SEO, it is not specific to content writing. There is also a Marketplace service for content writing, but again, it’s not specifically focused on SEO.
To gain access to the Marketplace, you must reach Icon level in the Partner program. You can find additional information here .