If you don not provide more information, we either cannot provide assistance or we have to make guesses as to the details you did not share. If I had not recognized you from previous posts that touched on this issue, I would not have any clue as to what you are asking.
I will guess that you have this relationship:
Project::__pkProjectID = Person::_fkProjectID
This could be a bad way to set up relationships, but I will use this to illustrate the basic concept:
If you open a custom dialog with input fields or use a popover (I'd use a popover), with a text field for the person's name, you could do this:
Make the text field for the person's name used in this dialog or popover a global text field.
This script can then be run when you click a button in the custom dialog or popover:
Set Variable [$ProjectID ; value: Project::__pkProjectID ]
Go To Layout ["Person" (Person) ]
Set Field [ Person::_fkProjectID ; $ProjectID ]
Set Field [ Person::Name ; AnyTable::GlobalNameFieldHere]
Go to Layout [Original Layout]
This is the simplest method for creating a new record in Person that is also linked to the current Project record. There are other options that work better, but this is a simple, beginner level approach. To create such a record without changing layouts in the script, do a web search for the keywords: FileMaker MagicKey.
And I suspect that you really need three tables, not two as I would guess that you need to assign the same person to more than one project and more than one person to any given project. If so, you have a many to many relationship and you need a join table between Project and Person to facilitate that many to many relationship.
Projects::__pkProjectID = Project_Person::_fkProjectID
Persons::__pkPersonID = Project_Person::_fkPersonID
Every time you add a person to a project, you create a record i Project_Person to link the current project to the selected person.