If 'ID' of your webElement changes everytime after refreshing the page in this situation:
First of all look for any other attribute which Is not changing every time In that div node like name, class etc.
Option 1: If this div node has class, name attribute then you can write XPath as below.
Option 2: You can use the starts-with function.
//input[starts-with(@placeholder,'Email or username')]
Option 3: You can use contains function. Same way you can use contains function as given below.