Display and edit a pre-existing text string?

I want the User to be able to edit a text string that they created at an earlier time. In order to do this, I have them click an EDIT button, which takes the text string and passes it to an EDIT PAGE, which contains a text field.

How do I get the text string to display in the text field when the page opens?
I tried <input type=“text” [(placeholder)]=“textString” etc. etc…,
But then when I type anything in the text field, the placeholder disappears.

How would I populate the text field with the text string in such a way that the User can click inside the field and edit the string?