Currently, it does not support to configure static IPs for VPN clients. Every time a P2S connection goes live, the next available IP is allocated in the address range. Refer to this.
In fact, the privateIPAllocationMethod is belonged to the ipConfigurations properties
in the VirtualNetworkGatewayPropertiesFormat object. What you need is to find the VpnClientConfiguration object, obviously, there is no such attribute to support the static IPs configuration for the clients. See vpnclientconfiguration in the template and the Azure REST API.
