Not possible without adding code (unless you are in a Web Form and have a conditional step with 2 different forms one with the mandatory and the other without it).
With JavaScript you add the 'required' class to the CSS of the element (to show the red asterix), then add a custom validator to put your logic for when it passes/fails the mandatory test. The code below adds a custom validator to validate that the emailaddress has been populated if email has been specified as a preferred contact method (as per https://docs.microsoft.com/en-us/powerapps/maker/portals/configure/add-custom-javascript
if (window.jQuery) {
(function ($) {
$(document).ready(function () {
if (typeof (Page_Validators) == 'undefined') return;
// Create new validator
var newValidator = document.createElement('span');
newValidator.style.display = "none";
newValidator.id = "emailaddress1Validator";
newValidator.controltovalidate = "emailaddress1";
newValidator.errormessage = "<a href='#emailaddress1_label'>Email is a required field.</a>";
newValidator.validationGroup = ""; // Set this if you have set ValidationGroup on the form
newValidator.initialvalue = "";
newValidator.evaluationfunction = function () {
var contactMethod = $("#preferredcontactmethodcode").val();
if (contactMethod != 2) return true; // check if contact method is not 'Email'.
// only require email address if preferred contact method is email.
var value = $("#emailaddress1").val();
if (value == null || value == "") {
return false;
} else {
return true;
}
};
// Add the new validator to the page validators array:
Page_Validators.push(newValidator);
// Wire-up the click event handler of the validation summary link
$("a[href='#emailaddress1_label']").on("click", function () { scrollToAndFocus('emailaddress1_label','emailaddress1'); });
});
}(window.jQuery));
}
