I'm trying to add a "SiteContact" entry to my "Site". When I Select the Site I am able to add SiteContacts however I would like the Site to be automatically selected when adding the detail (green box below). (it doesn't make sense to select anything other than the site i clicked on obviously)
Goto the SiteContacts Add Button Click Event-> Add a New Page Parameter(AddSiteContacts) CurrSiteID and pass the PagePropety SelectedSiteID. In the page load event of AddSiteContacts Page you can set the page parameter to required rID property.
First step you have done is correct. Now proceed to bind the site dropdown to PageSiteID. Alternatively you may keep the original binding[scaffolded page] of the drop down as it is [or remove the dropdown] you can set sitecontact.SiteID=${parameters.SelectedSiteID} as step 4 in page load event.