You may find it easier to take the stand off the bike, then fit the spring to the stand and the frame, then (still with both ends of the spring connected, lever the stand back into position .
OR
One trick for fitting a centre stand spring is to pack the spring coils with washers.
Hold one end of the spring securely (in a vice), and then bend the spring over, insert washers (or coins?) in the gaps between the spring coils.
With the washers in place, bend the spring over in the opposite direction and insert more washers on the other side of the spring.
This 'lengthens' the spring.
Add enough washers until both ends of the spring can just be hooked on (with the stand in the up position if possible).
With the two ends of the spring hooked on, pull the bike up onto the centre stand, if you are lucky that should stretch the spring even further and all the washers will drop out!
If the washers do not all drop out then you should be able to pull them out with pliers.