For what it is worth:
jacekt wrotethe problem is that my server cannot get IP with .1 in the last octet, because my server is behind another device which is a gateway for him
Presuming you mean server LAN is 172.16.101.0/24 and the LAN gateway is 172.16.101.1 then your OpenVPN server VPN subnet has to be something other than 172.16.101.0/24 .. the example given is 10.8.0.0/24 and the server IP is 10.8.0.1 and so there is no IP conflict.
jacekt wroteIs it possible to force customized server IP
Yes, manually expand --
server ..