Mail on iPad has no HTML signature field. The steps match iPhone Mail.
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Install it on a computer first
Install the signature in Apple Mail, Outlook for Mac or Thunderbird, then send yourself an email that includes it.
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Copy the signature
Open the email in Mail on the iPad. Press and hold a plain piece of the signature text (not a link), tap Select All, then Copy.
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Paste into Mail settings
Open Settings → Apps → Mail → Signature. On older iPadOS this is Settings → Mail → Signature. Choose All Accounts or Per Account, then press and hold to Paste.
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Fix the size
The signature may look too big. Shake the iPad, swipe left with three fingers, or tap Undo on the keyboard. Tap Undo Change Attributes. If shaking does nothing, turn on Settings → Accessibility → Touch → Shake to Undo. Compose a new email to check it.