To get update the product inventory in Simply CRM from Rackbeat, you can set up a Webhook to check rackbeat for product inventory at a certain frequency, such as daily. Remember that if you have many products, this process can become quite heavy, as each product will issue a Webhook call at this frequency, so do not do this too frequently, to avoid slowing down your CRM.
Using rackbeat typically means that you are also using E-conomic, which we will assume in this usecase.
Looking at Rackbeat's API documentation at https://rackbeat.stoplight.io/docs/api tells us that the endpoint to call for product updates is https://app.rackbeat.com/api/products which accepts number as a key and also the filter (to filter for products updated recently simple_filter[updated_at].
To create the webhook that will check each product, you do this by going to Workflows and add a new workflow for products.
Here you can add the Webhook action, which will send the number key to Rackbeat, where we will use eco_product_number (the product number which the CRM have synced from E-conomic) from the CRM, and as response we will store the stock quantity (products.0.stock_quantity if you check Rackbeat's API) to the standard CRM E-conomic quantity field called qtyinstock.
As always when working with API's its always advisable to just test the individual calls out in Postman, before setting them up in the CRM, as this process will help you debug any problems much faster.