Hi @Paul2000 ,
Do you want your single choice field display "choice1" as default value in new form?
Could you tell me whether do you use combo box or drop down to choose for this single choice filed?
1)If you use combo box
Firstly, unlock this field.
Then set the combo box's DefaultSelectedItems:
{Value:"choice1"}
Then, the combo box will display choice1 as the default value instead of blank value.

//in my test, my choice is "Complete","Incomplete".
2)If you use drop down, this will be a little complex.
Choice field display as combo box in default. Using drop down to update need to do more customizations.
Firstly, unlock this field.
Then, delete the original combo box in the choice field.
Insert a drop down to this field's data card.
Set the drop down's Items:
Choices(listname.choicefieldname)
Set the drop down's Default:
"choice1"
Set the datacard's Update:
Dropdown1.Selected
If you mind the error message of "Y" property, you could just remove the formula in it.

Then, the drop down will display choice1 as the default value instead of blank value.
Best regards,