Conference submission while article under review for journal
In economics it is entirely normal to submit a paper to a conference while it is under review at a journal. I write as an experienced academic economist who has done this (and seen it done) many times over the past 15 years. So it is fine for you to submit to the conference. No disclaimer/declaration is necessary.

The economics discipline has developed very permissive norms about this kind of thing out of necessity because acceptance rates at even mediocre journals are low (typically not much greater than 10%) and review times long. It's not unusual for a delay of 3 years to elapse between first submission to a journal and eventual acceptance at a (likely different) journal. Moreover, economics papers often take a long time to write and, as you correctly intuit, the most common practice is for economists to submit a paper to a conference only once a somewhat complete working paper is ready. If we were to insist on a strict sequencing of tasks then then result would be a decade-long cycle for a paper, whereas allowing overlap between writing, conference presentation, and submission allows work to be developed, circulated, and published somewhat more quickly.

All that said, you might like to consider why it is that you are submitting your work to the conference. An important reason, of course, is to disseminate your ideas. But another key purpose of conferences is that they provide an opportunity to obtain feedback that can improve your work. This is especially valuable the first few times you present, when you will quickly get an idea of what the controversial assumptions in your paper are, and what people perceive as the main weaknesses. It is often helpful to delay journal submission until at least after you have presented the paper a couple of times so you can figure out what issues the referees are most likely to raise and address these in advance. Given that you probably have to wait 4-6 months to get a set of referee reports back from a journal, it is often much more efficient to first get feedback on the conference circuit.