There is a video available below or the written guide here.

Please make a backup of your project before following this guide.

Start by going to the E_AttachmentType enum and click the new button to add a new entry then name it what you like. Then save the blueprint.

Next play in editor, this might take a while to load in the character as its re compiling the blueprints. Once the player loads in your can exit play in editor.

Next go to the BP_WeaponInventory and copy the video below.

Next go to the BP_AttachmentWindow and copy the video below.

Next go to the BP_WeaponInventory and select the NewAttachmentItem we created earlier and in the details set its AttachmentType to your new attachment type like the image below.

You have now added your new attachment slot. To add it to weapons you can go to there holdable blueprint’s class default and add the new attachment type to the available attachment slot like the image below.

You must also set the attachment blueprint in the weapons allowed attachments this is needed so that your new attachment can be added to your weapon, the Allowed Attachments are also in the Class Defaults of the weapons holdable blueprint.

You also need to set your attachment blueprint’s Attachment Type this setting is found in the attachment blueprint’s class defaults.

For more info on the settings attachments for weapon check out the guide below.