The federal oath of office should not be copied directly from the US., except perhaps for the stating of one’s own name.
So, I suggest instead a modified combination of the US, Russian and Swiss federal and solemn oaths:
I, [name], solemnly promise in exercising the powers of the President of The European Federal Union to fulfil these duties to the best of my abilities:
to observe and protect the Constitution of the Federation and the rule of law;
to protect the sovereignty, security and integrity of the Federation;
and to faithfully serve the people of the Federation.