Unable to remove textfield value in Ionic Angular

I’m Working on a project which is made in ionic and angular the issue i’m facing is whenever admin creates as user, user gets a signup link. when user opens a link a signup form shows then i’d applied all validations on it the issue is browser automatically fills the username field with it’s may be cache value but after clicking it’s get validated i want to validate it on page load but value is not present there it’s empty.
i’ve attache screenshots fro the same.

enter image description here

after clicking anywhere on page

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this validation works.

Anyone knows what can i do here. I want to show validation on page load if there is a wrong value