"We have to make travel and visa arrangements well ahead of the qualifiers so we need to know who is in.
"Those who fail to sign the document will be left out of my squad for the World Cup qualifiers."
Hey's first competitive game in charge of the squad will be a World Cup qualifier against Gambia at the end of May.
They also face Senegal and Algeria.
Meanwhile, Liberia will play Oman in an international friendly in Muscat on 24 April as part of the warm-up for the qualifiers.