You can try this:

$(".btn-outline-primary").click(function(){
        console.log($(this).prop('value'));
    });

To get the value of a tag , you could always use the value method.

How :

document.getElementsByClassName('btn-outline-primary")[0].value;

This will get the value of the first element with class btn-outline-primary

If you wish to get the values of all elements :

var arr = document.getElementsByClassName('btn-outline-primary');
var array = []
arr.map(_=>array.push(_.value))

In the case you are using JQuery you will need to add

.attr('value')

to get the value

You could find all buttons with active class and get their value

Something like:

var values = [];
$('.btn-outline-primary.active').each(function(key, value){
 values.push($(this).val());
});

values array will have all your values of active buttons

To get a value of clicked button you can do this. Just use the value property of the button to get the value associated with the corresponding button.

const buttons = document.querySelectorAll('.btn-outline-primary');

[...buttons].forEach(item => {
  item.addEventListener('click', event => {
    console.log(item.value);
  });
});

<div class="form-row">
  <div class="btn-group">
    <div class="col">
      <div class="form-group">
        <input id="MR" type="button" class="btn btn-outline-primary" value="MR" name="MR">
      </div>
    </div>
    <div class="col">
      <div class="form-group">
        <input id="CT" type="button" class="btn btn-outline-primary" value="CT" name="CT">
      </div>
    </div>
    <div class="col">
      <div class="form-group">
        <input id="PT" type="button" class="btn btn-outline-primary" value="PT" name="PT">
      </div>
    </div>
    <div class="col">
      <div class="form-group">
        <input id="US" type="button" class="btn btn-outline-primary" value="US" name="US">
      </div>
    </div>
  </div>
</div>

And if you want to store the value for later use then you can simply assign it to some variable.

const buttons = document.querySelectorAll('.btn-outline-primary');
let clickedValue;

[...buttons].forEach(item => {
  item.addEventListener('click', event => {
    clickedValue = item.value;
  });
});