How can I add text conditionally on my invoice template, to show only if the field is filled on the invoice?

e.g. if I put something in custom field 1, I want the text to show up, but otherwise not.

Try a script token with this;

Var := element valueForKeyPath:'parent.parent.reportData.object.project.extraField1'.

(Var == nil) ifFalse:
[
'Extra Field - ' ++ Var.
].