While surveys and comments can automatically send Google review links for positive reviews/survey responses, you can also send a link anytime you want.
From Comments Tab
If you are in the comments tab, you will want to click "Open Conversation":
Once you click "Open Conversation", you can then select "G Add Link" button:
From Conversation Tab
If you are already in the conversation tab, select the conversation you would like to send link to. Then, you can then select "G Add Link" button: