First of all, it is really important that you have selected "Mention Bot" under "Grant Permission" in Integrations -> Workplace for the bot you want to use for boosting. If you have not done this, the boosting will not work. Please check this before you boost a post. If you have not done it; select "Mention Bot" and press save in Workplace.
We recommend you to test the boosting in a small group the first time. Publish a post and ask one of the members to not read the post. Boost the post and check the results with the member who did not read the post.
Go to the Group in Workplace where the post is published
Press the "Bot" icon on the right side in the comment field, and choose the bot you want to use for boosting:
Type @ and the name of the bot, click on the bot:
To boost the post, press enter for both alternatives.