Explanation

Step 1: Enable field security in the Approval field.
Enable field security on one or more fields for a given entity.
Associate one more existing security profiles, or create one or more new security profiles to grant the appropriate access to specific users or teams (step 2 and step 3 below).
Step 2: Create a new field security profile.
Create a new field security profile for the sales manager.
Step 3: Set the field permissions...security profile
Step 2 and step 3, example:
Configure the security profiles.
Create the field security profile for sales managers.
Go to Settings > Security.
Click Field Security Profiles.
Click New, enter a name, such as Sales Manager access contact mobile phone, and click Save.
Click Users, click Add, select the users that you want to grant read access to the mobile phone number on the contact form, and then click Add.
Click Field Permissions, click mobilephone, click Edit, select Yes next to Allow Read, and then click OK.
Reference:
https://docs.microsoft.com/en-us/dynamics365/customerengagement/on-premises/admin/field-level-security