Having done that on my own site (www.hatchsolutions.co.uk) I found that one of the key "Gadgets" was missing .. the Contact Us form.
This was an immediate conundrum.. Sandbox Solutions do not allow email access so I knew I couldn't use that as an out, so I thought I would hunt down that Gadget.
STEP 1 - Add Office 365 namespaces to your Custom Master Page
This is a pre-requisite, as you will be using some of the "Office 365" web controls. I found these in the header of the "oob" root.master which you will find in the All Files section of your public website in SharePoint Designer.
Add these to the top of your Custom Master Page.
<%@ Import namespace="Microsoft.SharePoint.Spx.WebSite.Controls" %>
<%@ Import namespace="Microsoft.SharePoint.Spx.WebsiteCore" %>
<%@ Import namespace="Microsoft.SharePoint.Spx.WebsiteCore.Controls" %>
(note the special Microsoft.SharePoint.Spx namespace!)
STEP 2 - Create Custom Page Layout with Contact Us Control Embedded
This was the way I chose to do it .. you don't have to do the same (although I couldn't work out an easier way of doing this myself).
I created a new custom Page Layout (based from one of the others). I deliberately removed any references to any custom code-behind or code-blocks.
This means that I will be running OOB code, and won't be consuming any Sandbox Resource points!
Then add the following ASP.Net snippet into your page:
<iws:ContactUsControl runat="server" EmailTo="email@example.com" CultureName="en-US" SendToBCM="on" xmlns:iws="iws" />This is the same ASP.Net tags that get used when you drop the gadget onto your page.
You can see in my example I have added my own EmailTo and CultureName attributes .. you can set these to whatever is appropriate.
The other thing to note is that you must place this control in an EditModePanel with PageDisplayMode="Display". The Contact Us form otherwise tries to validate the page when you try and Save Changes or Publish (not good!)
And that is it .. simply save your changes, publish your page layout and create your own page! :)
I have created my own custom page layout here;
Please let me know how you get on with yours!