Hi @Kurt Stichelmans
Welcome to our forum!
What is the user's account? Outlook.com or something else?
Based on my research, you could add signatures on Outlook for web by going to Settings>Account>Signatures>Add your signatures>Save.
If you want to add more than one signature, click +New Signature after editing the previous one to create a new signature. You can also select the corresponding signature by dropping down the triangle to edit.
In addition, you could choose the signature you want to use for your new message or reply. More information: Create and add an email signature in Outlook - Microsoft Support When you compose a New Mail, you may add the signature by going to Insert>Signature>Choose the corresponding signature. Hope it helps!
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