Prince,

You can use 'value' attribute to set the value.

If is normal HTML form can user value.

If related to some calculated value or functional than can use t-att-value

Hope this might help you.

Regards,

AniL

Try to modify the default_get function, there you can set the default value, so when you load the page it show the desired value in the field.

Kind regards,

Juanvi

Edit the template of contact us from odoo back end and set value attribute for html element