1. Firstly, you will need to know the boundary points before proceeding in inputting the platform. You may go to Boundary >> move corner >> select the related boundary >> you can see the boundary points are circled in red circle.
2. These boundary points can be use as reference when you are inputting your platform and it makes is easier to locate the points.Please refer image below
3. Next, to get the platform to match the boundary line completely, you may need to use endpoint setting in you object snap function
4. Go to object snap function >> tick end point >> click OK (please refer image below)
5. Once the object snap setting is set, you may go to platform and start to model the platform and with this setting, you will get the platform nicely match the boundary without overlapping them.
6. Kindly note, throughout the modelling process, you may exercise care when it comes in finding the next endpoint so that the platform will not overlapped with the boundary.
7. You may refer this link for sample of how it is done. Please click link below