Yes, you can! It is now really easy to use Custom fields when creating new posts!

Custom Fields is designed to meet the needs of social media managers who wish to schedule identical content across multiple profiles with minimal customization. Using Custom Fields, you will be able to insert pre-defined values, such as dates, addresses, and signatures into posts and schedule them. As a result, you will save time and effort when scheduling a large number of identical posts.

1. Open up the Composer and select the Field picker > Manage fields > New field

2. You can now create your custom field and select if you want it to apply across either your:
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If you use this option it will allow you to add a different value per social profile.

Using this option allows you to use the same value for a set of social profiles

Finally selecting this option allows you to use some value that you put in all of your posts, no matter the social profile

Simply start composing your post and use the Field picker and select the desired field when needed. As shown in the short GIF below:

1. Open up the Composer and select the Field picker and select Manage field

2. Select the variable you want to manage and then make the required changes or delete the field.