To install the email signature in your Outlook account for iPhone, please follow these steps:
Click on the 'Copy HTML' button in the Install Signature popup to copy your signature code.
In Outlook iPhone app, Tap the menu button/account picture on the top left.
Tap the 'Settings Gear' icon on the bottom left of the menu.
In the Settings menu, Tap on the signature section.
Clear everything in signature box and Double tap in the signature field box and select Paste to insert your signature.
That's it. Compose a new email to test your signature.