Is your dogs already clicker savvy? If she's already picking up the toy for retrieving, mark the act of her picking up the toy & reinforce right away. Reinforce that, not when she brings it back. When she starts offering this over and over, add your verbal cue just as she is getting her toy. Vary the location. Then try your cue to elicit the behavior if you think she's made the association between the cue and picking up the toy. If not, keep using the cue while she's picking it up, not before...for a while more. Your cue can even include the name of the toy. Once she gets onto the game, you can name other toys. Be sure to vary locations and contexts, your body position too so sh can separate other things she might construe as cues...get it on stimulus control. Be sure to use something she loves. So she doesn't lose the retrieve, do some of those by themself at another time. Use a different cue. Don't reward for the wrong behavior... Like if she mixes those two things up.