You can write like this:

Guarantee letter of commitment (letter)

I, XXX, voluntarily provide repayment guarantee for XXX's mortgage loan from XXX (house) to XXX (bank) until the loan is paid off.

Guarantor (signature):

Date:

Note: the guarantee is not signed casually. You should consider the liability of guarantee.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.