Okay, so I am pleased with the result – now comes the hard part!
Before starting, 3ds Max uses channels at the time to make UV Maps; default textures are always set in Channel 1.
What we need to do now is to join both elements into a single object, without losing their materials. So, right-click on the teapot, Convert and Convert to edit poly (Fig.05).
Click on Attach and select the plane to join both objects. Select the "Match Material IDs to Material" option and this will make our object have 2 different materials in a single mesh (Fig.06).
Now, our object is only one, so the next thing to do is to apply a "UVW unwrap" modifier. Let's go to the Channels option and change the values to Channel 2. Then we click on Edit to open up the dialogue, Edit UVWs (Fig.07).
Inside the UVWs dialogue, we click on the face selection mode to activate the option: "Mapping > Flatten Mapping"
Inside the dialogue we put the following values (it is very important to remember all settings used!) (Fig.08a).
We should now have this result (Fig.08b).
The next step is to open the Render to Texture window, found in Rendering; in the tab "Mapping Coordinates" we'll change the Object option to Use existing channel, and we'll select Channel 2 to make the texture rendering be distributed in the same way as our map. In Channel 2, where all faces have one part in the texture, we click the Add button and select CompleteMap. Last but not least, we choose the size of our texture, and in this case 512x512 is enough for us (Fig.09a – b).