Are you sure its a crack not just a scratch? Cracks from movement at usually in the corner welds
Very odd place for a crack, If it is I would say its an impact crack from having been hit by something.
The best way would be to get out a "magic man" company but would probably cost you £200ish.
The cheapest and easiest way would be to clean it out with some solvent cleaner, Masking tape around it and then smooth in some white silicone, Silicone will get dirty though. Could use Stalmax but its tricky to work with and would go yellow eventually.
Final option would be to get a PVC wax repair kit for about £50 but it is an artwork job to use them. I have tried many times and cant get it to look perfect.
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