How to assign a specific private ip (pre-determined ip base on my CIDR and subnet) when create ec2 using Cloudformation script

This the code I use, still not working. I am trying to create the instant using CloudFormation template and to be launched with specific private ip address. Please help what am I missing : where would I mention the private ip that I need it to be assigned to the instant in the CloudFormation template?

"Ec2Instance" : {"Type" : "AWS::EC2::Instance","Properties" : {"ImageId" : { "Fn::FindInMap" : [ "RegionMap", { "Ref" : "AWS::Region" }, "AMI" ]},"KeyName" : { "Ref" : "KeyName" },"NetworkInterfaces": [ {"DeviceIndex": "0","PrivateIpAddress" : String,    } ]  }}```