Hi @Dive1,
Let me make it easy for you, you just want to save the previously input car immat as the dropdown list, right?
Not sure what is the column type you select to save the car immat, Choice or Text?
If it is a Choice, please edit the column setting as below to make it add values manually.

Then in your form, please modify the Update property corresponding to the car immat data card as below:
If(
!IsBlank(ComboBox.SearchText),
{
Value: ComboBox.SearchText
},
ComboBox.Selected
)
If it is a Text column, please check as below.
I set the ComboBox Items property as below:
Distinct(List,CarImmat)
Modify the Update property corresponding to the car immat data card as below:
If(
!IsBlank(ComboBox.SearchText),
ComboBox.SearchText,
ComboBox.Selected.Result
)​