In your main project under Code\IISMeta.wxs you'll find the markup that defines the IIS configuration. This file can express any valid WiX IIS meta as documented here:
https://wixtoolset.org/documentation/manual/v3/xsd/iis/
A basic example (needs some tweaking) would look something like this:
<Directory Id="webSites" Name="WebSites">
<Component Id="webSite" Guid="PUT_GUID_HERE" KeyPath="yes" Permanent="yes">
<CreateFolder />
<iis:WebSite Id="DefaultWebSite" SiteId="*" Description="Default Web Site" Directory="webSites" ConfigureIfExists="no" >
<iis:WebAddress Id="webSite" Port="80" />
<iis:WebDirProperties Id="webSite" AnonymousAccess="yes" WindowsAuthentication="no" />
</iis:WebSite>
</Component>
<Directory Id="webSiteUi" Name="UI">
<Component Id="webSiteUi" Guid="PUT GUID HERE" KeyPath="yes">
<iis:WebAppPool Id="webSiteUi" Name="WebSiteUI" Identity="networkService" ManagedRuntimeVersion="v4.0" ManagedPipelineMode="Classic" />
<iis:WebVirtualDir Id="webSiteUi" Alias="Something" Directory="webSiteUi" WebSite="DefaultWebSite">
<iis:WebDirProperties Id="webSiteUi" AnonymousAccess="yes" WindowsAuthentication="no" />
<iis:WebApplication Id="webSiteUi" WebAppPool="webSiteUi" Name="Something" />
</iis:WebVirtualDir>
</Component>
</Directory>