From your line items page, create a new line item for this work (you could also duplicate an existing line item if this has a similar description, pricing structure etc).

From the pricing tab, set the rate as the price per hour. Decide whether this rate should be monthly (ongoing work) or one off (ad hoc project).

Add a calculation to this fixed rate to multiply the hourly rate by a quantity field. You can name this quantity field something like the 'estimated number of hours.'

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This will allow you to estimate the number of hours you will spend on this when creating a proposal.

Alternatively you can skip this step and just include the hourly rate within the line item label and leave it as 0 in the table of fees.

Either way, in next step we will edit the proposal template so your client knows this fee will be billed hourly upon completion of the work when the actual number of hours spent are assessed.

Duplicate an existing proposal template to create a template for this type of work and make sure this is included in the proposal name.

Use the please note section to provide all the information your client needs about your hourly services.

Save this down and head to Step 3.

When creating a proposal and completing the details, you can edit the please note section on the fly on the create a proposal page. This means you can specify which services are to billed hourly for that proposal if needed.
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With this in mind, you can then change the totals for those line items on the calculation page, or leave them as they are, ensuring you add any explanation of what the future costs may be.
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Please don't forget that if you do this, you will need to invoice and bill your client outside the program once you know the total hourly fee that you will be charging them.