Having the homepage only redirect to another page

Hey! I am struggling to figure out what to do here. I have Webflow to build out a series of pages for a client with an existing website. The main domain needs to link to their existing website, and the rest of the subdomains need to be kept intact. People would usually be directly linked to these subdomains at least for now so I don’t think that discoverability of the subdomains would be that important, but I’m worried about how different types of redirects only for the main page would affect the SEO of the client’s existing website. I am also unsure how to do this technically as they have elected to host with another provider and as such I don’t have a hosting plan through Webflow. Even if I had access to 301 redirects that seems as though it’s for the entire domain versus just the main page.

Unfortunately due to agreements signed I am unable to share the read only link to the pages I’m building, even though there’s not much anyone could learn about the project by seeing it.