To get value form any input field at first new need to select the field with CSS selector and document.querySelector()  Then selectedField.value to get field value

Example: Get value form email field of form: https://leomeasure.store/contact/

var emailField = document.querySelector('form.uagb-forms-main-form input[name="f7e0adcb"]');
var email = emailField.value;

Same way we can get full name: 

var fullNameField = document.querySelector('form.uagb-forms-main-form input[name="3e51448a"]');
var email = fullNameField.value;

To get this value as GTM variable we need to crate a variable with custom JavaScript 
Example GTM JS Variable Code:

function() {
    var emailField = document.querySelector('form.uagb-forms-main-form input[name="f7e0adcb"]');
    var email = emailField.value;

    return email
}