How to add checkboxes in microsoft crm 2015

I have a form. This form includes the information of the main firm. This main firm has a lot of firms such as 40-50 child firms. These 40-50 firms belongs to the main firm. I want to add a textbox named firms. When i clich to the text box, a small window will appear which includes the child firms that are belongs to (their main firm is parent firm) the main firm. Each child firm is especially a check box.

I mean in this second opened form. checkbox A (Assume that A is the mane of the child firm)

How can i do that can anyone help me please?