The article https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Customizing_the_Splas... states that the <a> tag is acceptable in a custom splash page. However, if I put in
<a href="https://www.mysite.com">My Site</a>
I get the link and it works, but it's just blank space, no text. So, it's not obvious that the link is there unless you hover the mouse cursor over the link and notice that the cursor changes.