[Plugin: Advanced Custom Fields] Username as radio button value

I am making some kind of volonteering management system for the local television network and I want the volunteers to assign to different jobs.

For example, on Monday there is a big sportsevent and I want volunteers to make an newsitem. I need 3 different people:
– Cameraman
– Audioguy
– Reporter

I use Advanced Custom Fields to add the radiobuttons to the eventpost. When a volunteer is signed in he can check one of the radiobuttons if he wants to assign to one of the jobs.

So the value of the checked radio button has to be set to the current userlogin.

Can someone please help me out? I am trying to get this for days.

https://www.ads-software.com/extend/plugins/advanced-custom-fields/