Allow Clients to express interest in candidates when making a shift job request

Navigate to More → Global Settings

Select Jobs → Shift Job Settings → Client Side

Scroll down to find the setting 
○ Client can express interest in candidates when making a shift job request

Enable this setting and make sure to hit the save button located at the bottom right side of the screen. 

Once enabled, this should allow clients to see a list of available candidates and select the ones they want to express interest in.