Set Lookup Field Disable/Enable in Dynamics 365 CE Portal

Here is the snippet which helps to set the lookup field as Disable(Read-only) in the dynamics portal.

function disableLookupField(lookupId) {
    $('#' + lookupId).parent().find('.input-group-btn').hide();
    $('#' + lookupId).parent().parent().find('.input-group').css('width', '100%');
    $('#' + lookupId + '_name').prop('disabled', true);

}

Here is the snippet which helps to Enable the lookup field in the dynamics portal.

function enableLookupField(lookupId) {
    $('#' + lookupId).parent().find('.input-group-btn').show();
    $('#' + lookupId + '_name').prop('disabled', false);

}

Set a field Mandatory or Non-Mandatory in Dynamics 365 CE Portal

If you want to make field mandatory on Dynamics 365 CE Portal Form using javascript/JQuery

var MakeRequired = function (fieldName) {
    try {
        if ($("#" + fieldName) !== undefined) {
            $("#" + fieldName).prop('required', true);
            $("#" + fieldName).closest(".control").prev().addClass("required");

            // Create new validator
            var Requiredvalidator = document.createElement('span');
            Requiredvalidator.style.display = "none";
            Requiredvalidator.id = fieldName + "Validator";
            Requiredvalidator.controltovalidate = fieldName;
            Requiredvalidator.errormessage = "" + $("#" + fieldName + "_label").html() + " is a required field.";
            Requiredvalidator.initialvalue = "";
            Requiredvalidator.evaluationfunction = function () {
                var value = $("#" + fieldName).val();
                if (value == null || value == "") {
                    return false;
                } else {
                    return true;
                }
            };

            // Add the new validator to the page validators array:
            Page_Validators.push(Requiredvalidator);
        }
    }
    catch (error) {
        errorHandler(error);
    }
}

If you want to make field non-mandatory on Dynamics 365 CE Portal Form using javascript/JQuery

var MakeNotRequired = function (fieldName) {
    try {
        if ($("#" + fieldName) !== undefined) {
            $("#" + fieldName).closest(".control").prev().removeClass("required");
            $("#" + fieldName).prop('required', false);

            for (i = 0; i < Page_Validators.length; i++) {
                if (Page_Validators[i].id == fieldName + "Validator") {
                    Page_Validators.splice(i);
                }
            }
        }
    }
    catch (error) {
        errorHandler(error);
    }
}