Updating a sharepoint column

[Martindeter]

Hi all, 

I try to create a visitor management application. 
I created 4 screens :
-Welcome screen (visitor clicks on "visiting" or leaving")
-Visitingscreen (visitor fills in fields and clicks on submit to create an item on a sharepoint list (VisitorList))
-Leavingscreen (visitor fills in Surname field and clicks on submit to update a column (CheckOut DateTime)
-Thanks screen

VisitingScreenLeavingScreen

I don't know how to write the formula for the submit button on the LeavingPage. The Surname should match with the surname on the sharepoint list and update the right column. 

Could someone help me ? 

Thanks !!

[Martindeter]

Up. 

[Martindeter]

I wrote this function for my Submit button. I would like to replace "pp" by the surname field but it doesn't accept. 

[skmcken]

Use the ID field of the List: - 

Patch('Visitor List',

{ID:FormSelectedRecord.ID,

CheckOut:Now()})

[Jeff_Thorpe · Accepted solution]

@Martindeter,

In your last screenshot try replacing "pp" with TextInput.Text

The "TextInput" is the name of the surname control on the screen.