The example is more instructive, if it is split between two machines and follow the two step setup. Do not try to automate the communication by a script, it's erronous and not instructive. Remember to always use documentaion IP addresses (RFC 5737)

In order to connect two machines (A and B), create an interface first:

A# ifconfig wg0 create wgport $port wgkey "$(openssl rand -base64 32)"
A# ifconfig wg0 inet 192.0.2.1/32 alias

and on the other one

B# ifconfig wg0 create wgport $port wgkey "$(openssl rand -base64 32)"
B# ifconfig wg0 inet 192.0.2.2/32 alias

Then obtain the relevant information from the interface output

A# ifconfig wg0
... show the example output ...
A# ifconfig em0 inet # external interface
... show the example output ...

Now you need to communicate the the external IP, the wgport and the wgkey paramaters to your peer.
Each party finish the setup by supplying the peer's information.
A# ifconfig wg0 wgpeer "peerkey" wgendpoint "externalpeerip" "peerport" wgaip "peerip"
and
B# ifconfig wg0 wgpeer "peerkey" wgendpoint "externalpeerip" "peerport" wgaip "peerip"

Now you can ping to the other side.
A# ping "peerip"

Please add the description of the parameters (wgkey, wgport, ...) or point to the appropriate part in ifconfig(8) man page.

@donner I added the ifconfig options as requested, they are different in comparison to OpenBSD, so thanks for pointing this out.