This is the document for adding custom control :
https://learn.microsoft.com/en-us/azure/devops/extend/develop/custom-control?toc=%2Fazure%2Fdevops%2Fmarketplace-extensibility%2Ftoc.json&view=azure-devops
and you can make it hidden from process change as in this link :
https://learn.microsoft.com/en-us/azure/devops/organizations/settings/work/customize-process-field?view=azure-devops#hide-a-field-or-custom-control