Pair builder

Open Tools › Pair builder or go to /tools/pair. With an asset chosen the address is /tools/pair/<id>.

Steps

Pick an asset in the list on the left, search by ticker or name.
In the markets table tick one market in the Long column and one in the Short column. Use Market type to narrow the list.
Click Open spread.

The same rules as in the scanner apply, and the page explains a refused choice:

Message	Rule
Pick two different markets	The same market cannot be both legs.
Spot can be the long leg only (or spot vs spot)	A spot market cannot be shorted. Spot against spot is allowed on two different exchanges.
Margin can be the short leg only	A margin market is a borrowed coin sold on spot.
Both legs on one exchange are not an arbitrage	Two markets of one exchange with the same quote do not make a pair.

The resulting address follows the spread link grammar, so you can also type it by hand.