Weight loss is not listed as a side effect or a consequence of taking Belsomra and neither is weight gain. If you are taking Belsomra, then you should not expect your weight to change much at all. Weight loss with Belsomra was not reported in clinical studies but a small study that gave Belsomra to people with type 2 diabetes and insomnia did report a reduction in waist size, but weight loss was not significant.
If you do notice weight loss with Belsomra it may be the result of reducing your calorie intake because you are now sleeping through the night and not getting up for snacks.