Hi,
You can't select B1s from the portal. What you can do is use Azure CLI instead. For example, you may use Azure Cloud Shell to execute below command:
az mysql flexible-server create --location westus --resource-group MyResourceGroup --name myserver --sku-name Standard_B1s
Please see documentation below for az mysql flexible-server create command:
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP