Hi @Hoc ,
Do you want to add a new item if there's no matching text with the text you enter in this combo box?
If so, you could use ComboBox1.SearchText to represent the text entering in the combo box and use this to compare with the items in the combo box.
I've made a similar test for your reference:
1)set the combo box's Items:
list1
//my data source name, please replace with yours
2)set the combo box's OnSelect:
Set(var,ComboBox2.SearchText);
If(IsEmpty(Filter(list1,field1=var)),Patch(list1,Defaults(list1),{field1:var}))
Then if the text in the combo box has no matching text, it will create a new item with this value automatically.
Please notice that the combo box's OnSelect will not act when you are entering text in the combo box.
You need to firstly enter data in the combo box, then click the blank space, then click the combo box again, the onselect property will be triggered.
Like this:

Best regards,