A /31 subnet is a tiny subnet consisting of just 2 IP addresses and no broadcast or network address. Defined by RFC3021 a 31-bit prefix is designed specifically for point-to-point links, which is about the only application that can cope with having no broadcast or network address.

31-bit prefixes despite existing for a long time are not that well supported, most the major vendors such as Cisco and Juniper support them without issue but smaller vendors such as Mikrotik don't explicitly understand the concept, that is not to say however that it won't work.

Adding a /31 IP address to an interface on a Mikrotik Routerboard is simply a case of adding one of the pair of IP addresses as the interface address with a /32 mask, and the other IP address in the subnet as the network address.

In the command line this can be done like this:

/ip address add address= disabled=no interface=ether1 network=

This can also be easily accomplished in the GUI in a similar fashion but filling in those values to the relevant GUI fields.