Hello,
Welcome to Microsoft Q&A,
You will be able to create a deep link to the overview page of the API, and currently, we don't have the provision to take up directly to the Design page.
https://portal.azure.com/#@<tenant_id>/resource/subscriptions/<subscription_id>/resourceGroups/<resource_group>/providers/Microsoft.ApiManagement/service/<apim_instance>/apis/<apiName>/design
Please Upvote and Accept the answer if it helps!!