Saving name from a text field to a people/group column, how?

I've been able to do something similar in the past by using a SP designer workflow. I was capturing the users email via plain text and then copying that into a person field via an Update Item step.

Not certain if the same logic will work with a plain a First and Last name, I would think it would. One thing I would look out for if you go that route is if it's a large company with two "John Brown"s the workflow may have trouble resolving the name and error out. 

If you have Flow, you may also want to look into the Resolve Person action. It seems like it would work the same way. 

https://www.chakkaradeep.com/2018/03/14/validate-sharepoint-person-field-in-microsoft-flow/